I personally don't think any chemical warfare would be effective against the flood. It adapts too quickly, assimilating any thing and everything it comes into contact with. If allowed to to reach gravemind state it becomes powerful enough to outthink A.I. and control technology. What happens if it takes control of droid making facilities in the Star Wars universe?

Why wouldn't chemicals be effective? If you pour acid on something, at the very least it's not gonna be fine. IF allowed to reach gravemind state it could be dangerous. But only IF. I don't think it would get that far. The Star wars universe could starve the flood out by fighting it only with droids. Then ships could blockade the planet. Orbital bombardment would be just as effective as glassing if not better.

But the Star Wars universe is so fractured, by the time the flood would be seen as a threat it would have space traveling technology. That is why the forerunners eventually lost. The humans were attacking planets trying to stop the spread of the flood and the forerunners thought they were being aggressive and greedy. The flood needs only one quiet infection of a planet with space faring technology. After that they can travel any where. If they start in the fringes of the galaxy and take a few planets not part of the republic, Game Over.

Cept I highly doubt any space faring planet would be taken over easily. Every planet that has space fairing tech, is under the Republic or the Empire. Since this scenario counts both sides working together, they entire galaxy is united under them. That means any tech is going to be apart of some organization that would be raising flags as soon as the flood got onto the planet. The flood may take the planet, but they would never be able to leave the planet. The blockades would stop that. Plus Hyperspace would not work as any blockade fleet is bond to have a few hyperspace jammers.

Fair enough. The flood would only need to get to blockade ships not beyond them. It spreads quickly. Any planet the land is lost anyways. No one can ever leave it. Wasting man power on blockade also. And let's face it, people are stupid. How long to some scientist wants to study the flood? How easily would it spread as a first the republic does not recognize it is more than a sickness but a sentient life form? How many planets would be lost? How could they keep track of every ship that left before the blockade? The forerunners lost for a reason. They treated the flood as a sickness to be quarantined not fought. Undoubtedly the republic would respond the same way. And the forerunner did this for hundreds of years before realizing their folly. Could the republic fight the flood for hundred of years? Would they win a battle of attrition if the flood infected say "Couresant"?

My answer is, maybe. You're argument admittedly leans on pure speculation. Could the flood win? Yes. Could they not win? Yes. A problem I see with your logic is that you treat the Republic like they are the Forerunners. They aren't too similar. Now throw the Empire into the equation and the whole scenario can be changed. Instead of a blockade, the Empire may just destroy the planet and everyone on it. Therefor the flood goes with it. The Empire would do whatever it takes to kill the flood. Just like how they reacted to the Yuzong Vong. Which the Yuzong Vong would also be on their side too. That is just a recipe for no survivors.