Depends really. I don't see the Hive banding together with the fallen, Cabal or Vex. I see the hive as pretty mindless agressors. They'll attack anything they see that isn't hive. Think zombie. (Because they are known as space zombies) They could be a virus that only wants to spread. But, then again... I don't think that is what they are. I see them more as insects. Being controlled by a hierarchy. So you have the mindless attack drones, but then possibilty a smarter one giving the orders. As for the Vex, I don't see them pairing up with anybody. I don't see a hierarchy within the Vex either. Just a bunch of robots, perhaps bigger robots that are harder to kill, but they have the same directive as any of the smaller ones. I think the Vex will kill anything that they deem a threat. (This goes into my theory on where the Vex came from, which is the future.. I think their master is The Traveler, and they went looking for him when he disappeared, and they found him hovering over the last city, half destroyed, where you, the guardians are, so I think the Vex want to kill you for that reason) The Fallen and Cabal however are free thinking. Which means they can pair up with each other if it's mutually beneficial.