The only problem with this is that everything is allowed. That being said, the Flood and Hive are eerily similar, especially to the Taken. It would be likely that they would sort of team up, since neither would be able to take the other over completely. And as for mechanized enemies, well. I'm pretty sure that the Vex would be able to time-loop them or something until they figured out how to bypass any of their defenses. Sorry to say it, but Destiny's lore is a bit more powerful than Halos. Speaking of which, those only affect what is alive. So, the Hive would probably be fine. Unless it kills their worms, then they might have a problem.