The original Halo array had 12 Halos which fired linearly, and could be tuned to change their area of effect as well as what sorts of beings they'd affect. There were 12 because the Forerunners originally thought that these would be able to combat the Flood without having to purge the entire galaxy, which is what the second set of Halos did.
The second Halo array had 7 Halos which fired in the 25,000 lightyear radius sphere as was stated before. This Halo array required at least 7 to cover the entirety of the galaxy to be sufficiently sure that everything died. (as a technicality, 6 of these Halos were created by the Lesser Ark, while one of them was the sole survivor of the previous array, though I believe it was modified to function more like the latter Halos.
Regarding the power of the Flood, I'll start with the TLDR version, then give my full answer.
TLDR version: The Flood are the hammer of a god-race who decided that the Forerunners were a nail that had to be pounded flat. The Flood had the ability to consume all organic matter, absorb the intelligence of all sentient beings, and command all technology they're exposed to, meaning that everything the Forerunners did ultimately strengthened the Flood instead of weakening it. But the Flood's ace in the hole was when it used Neural Physics (the Star Roads in particular) to crush the Forerunners utterly.
I've read through the Forerunner Saga a few times, and I've come to some conclusions regarding Halo's ancient history (particularly the Precursors and the Flood), which may not be apparent on the surface. So to give my explanation of the Flood, I'll give a bit of context.
The Precursors exist outside of space and time, and possessing unfathomable power. They created organic avatars which they then used as agents of their will to interact with their other creations. They then created a sentient, organic race they intended to give the "Mantle of Responsibility" to. However, they did not bestow the Mantle immediately, they allowed the race they created to develop naturally without Precursor interference. This resulted in the race developing into two distinct races. The first remained close to their genetic stock, and though they sought to optimize their potential, they didn't fundamentally change what they were. This first faction is humanity. The second faction actively sought to turn themselves into the galaxy's ultimate species, drastically changing their genetics, copying favorable biological traits from the other species and incorporating them into their own. This second faction became the Forerunners.
The Precursors eventually clarified to the Forerunners that they would not be inheriting the Mantle, and that the Precursors would instead give the Mantle to the other faction of the species they'd created, due to humanity still being what they were designed to be. This threw the Forerunners into a rage and they lashed out as hard as they could, using their every tool of war to try to exterminate the Precursors. However, as I said in the first line of this history, the Precursors exist outside of space and time, so the Forerunners have no means of actually harming them. So that means the genocide the Forerunners unleashed was on the Precursors' avatars, since those were the only representation of the Precursors the Forerunners had access to, and it doesn't appear that the Forerunners understood the distinction between the Precursors existence as constants, and their temporal organic forms. These avatars not ultimately being important to the Precursors' existence explains why they didn't bother to defend themselves. And they very clearly chose not to defend themselves, because if they'd weaponized the Star Roads, the entire Forerunner Empire would have fallen, even without the Precursors using any other abilities or technology, due to the Forerunners being completely incapable of harming those Neural-physical structures.
Now as far as I can tell, the Precursors' avatars seem to have some agency of their own, having motivations and limitations on their knowledge and abilities which do not apply to the Precursors. More on that later.
The few surviving Precursor-avatars preserved themselves in one of two ways, turning themselves into dust, or placing themselves in temporal stasis. Over time, both of these turned into what we now know of as the Flood. The Flood's narrative is that this was an unforeseen change which it approved of. However, I believe the creation of the Flood was part of the Precursors' plan, though it is possible, that the Graveminds weren't made privy to this knowledge.
One of the Precursor-avatars which was in stasis is eventually found on Charum Hakkor by humans who use it as an oracle until it gives them an answer to their question about the Flood that is so terrible, that those who heard it committed suicide rather than live with that knowledge. This is of course, the Primordial, the first Gravemind, still in the form of the Precursors' final avatars, being composed of one being, rather than the bodies of many.
The now-Flood dust was placed in jars on ships, and sent across the great void of space from Path Kethona to the Milky Way, where they've only been found on worlds under human control, implying a degree of foresight on the part of the Precursors. Humanity inevitably experimented with this dust, and accidentally set the Flood loose on the galaxy. I believe this was a significant test of humanity's character, contrasting them with the Forerunners. Facing the responsibility of having potentially doomed the galaxy, humanity chooses to self-sacrificially do everything they can to save the other races from the horrors of the Flood. This culminates in an attempt to defeat the Flood by sacrificing a third of their entire population to create an anti-Flood. The Flood eventually reveals to us that this attempt was not successful, and that the Flood willfully left to create the illusion that humanity had succeeded in creating an immunity. The stated purpose of this was so humanity would survive, and to prevent the Forerunners from exterminating them. Along with the Flood's promise to make sure humanity out-survived the Forerunners, it also told them that it would return, and they would be tested again.
This brings us to the Forerunner-Flood War. The Flood could convert any organic matter they came into contact with into Flood biomass. The Graveminds could turn AI against their creators, causing them to serve the Flood, just by talking with them. The Flood was capable of using all Forerunner technology, as well as any other technology they encountered. The Flood even had access to Precursor technology and knowledge. The Forerunners were not able to stop the Flood even when they stopped glassing worlds in favor of blowing up the stars of infected systems (which they eventually stopped in favor of the Halo Array).
The Forerunners actually calculated that the Flood could have consumed everything in the entire galaxy in "a few hundred years", and that was using only the Flood's expressed abilities before they started manipulating reality with neural-physics or even activating the neural-physical constructs like the Star Roads, which if used would have drastically reduced the amount of time the Flood needed to consume everything. But in spite of this, the Flood took thousands of years from first conflict until they finally started weaponizing the Star Roads, giving the distinct impression that they were either toying with the Forerunners, or waiting for something. I believe it is the latter.
The Forerunners would never have been able to purge the galaxy of Flood without the Halos, and the Halos' neural-physical properties would not have been possible without the technology stolen from Charum Hakkor, which was where the Primordial had been helpful to humanity. So it is very possible that the Precursors may have ordered events to equip the Forerunners with their final weapon, which ultimately the Flood would manipulate to end the Forerunner civilization.
The Flood was absent for just long enough for the Halos to be designed and built, and then it attacked with just enough fervor to show that the original Halo design wouldn't work, and once the second Halo array was completed, the Flood stepped up its game, and pushed just hard enough to reach the Greater Ark (where the Forerunners were trying to preserve only their own civilization, going so far as to remove other races which had already been put there for safekeeping) and destroy it before the 2nd Halo Array was fired, leaving the Lesser Ark with the rest of the saved races in peace to repopulate the galaxy. So it could be argued that the Precursors (through the actions of the Flood) allowed the Halo array to be fired.
This would allow the Precursors to wipe evidence of their existence from the galaxy, leaving future races to stand on their own, without relying on the god-like Precursors to solve their problems. It gives the Flood an out, allowing it to be gone to allow the other races to re-surge, without making it obvious that it willingly let life continue. This also allowed the Flood to be used to a lesser extent to manipulate galactic events by serving as a villain and enemy to all, without it's true power (which these modern races could never hope to match) ever being revealed.
Hopefully this was pretty easy to follow. If not I can explain, or give quotations. To my knowledge, this doesn't contradict any confirmed canonical information.
Was it R'tas who said that a single Flood spore can destroy an entire planet?
The only known effective way to counter the Flood is by starving them of hosts and thus destroying all sentient life within reach. All other methods risk survival of at least a few Flood spores which, in turn, can start the whole shabang all over again.
The quotation was that a single flood spore could destroy a species (multiple planets or even star systems). I do believe it was R'tas, though it was in a Halo 3 cut-scene, so it should be pretty easy to find.
To clarify regarding the Halos, the first array would likely have not killed the Flood, being as it required an additional solvent to get rid of the bodies when fired, and the Flood's undead nature means it could have likely rebuilt itself. But that's why the second Halo Array completely wipes neurological beings from existence (as seen in Halo Fractures). The Forerunners couldn't risk this not working, so I suspect they continued with their plan to save as many species as they could on the Lesser Ark, while completely wiping the galaxy of life, so any surviving molecules of Flood wouldn't have anything to consume, and would hopefully die.
Pretty sure the rings only take out a quadrant of the universe.
As for your last question, the Halo's were a Last Resort plan. The weapons were top secret to all but a few people, so the flood didn't know they were building them until later when Mendicant Bias went rampant. Plus they were built outside the Galaxy. A Last Resort, if you don't know, is if all their other plans fail. However, they managed to only take out a few of the 30,000km rings. Except 07. However, if Installations 08-12 were to be fired. It would definitely have got rid of the flood. As I'm sure the original Gravemind moved out of the quadrant and far far away. If not, he died.
But remember, there were 12 Halo Rings, 5 of them were destroyed. And I believe 07 was deactivated.
If I may clarify, The Forerunners created two Arks, and a total of 18 Halo Rings.
They created the Greater Ark, which created the first 12. Then they created the Lesser Ark, which created the latter 6 (which fired in conjunction with Zeta Halo, the sole survivor of the original array).
The second Halo Array had a known maximum range. The original 12 however fired linearly, and though they didn't cover the galaxy as completely as the latter set, their range was effectively unlimited stretching through space to other galaxies, far beyond what the Forerunners could dream of ever seeing.