I'll give you the Mandos. The zombies (The two that come to mind are the Geonosion ones and the ones from Death Troopers) would just go into a stalemate with the flood. They would both be exchanging infected. The Vong have more in numbers (They outnumbered every faction combined in the Star Wars universe). Getting to the rings wouldn't be an issue considering they have the support of every other fleet in the Star Wars universe. I was thinking that they would just render the ring useless while terraforming it.