>>713572932No
The flood can't infest xenomorphs, the acid blood hard stops infection.
The flood MIGHT be able to use dead xenomorphs for biomass, once sufficiently exanguinated, but this leaves the Flood in the situation where they have to use purestrains, which means they can't both fight and form gravemminds. Same exact issue the Flood runs into vs Tyranids, except even worse, because this isn't a round 2 adaptation, this is just what Xenomorphs do.
If the Flood are already fully established, fine, they fuck reality and everyone dies.
Flood vs Necromorphs is a lot harder. At a baseline, both are kinda fucked at the start, since both need a kick start of biomass to get rolling, that the other isn't providing. If we assume this starts with the unluckiest motherfuckers in the universe ever opening two vaults, one full of both, Necromorphs have the speed advantage, and are more resourceful, but Flood can commandeer any weapons or technology around them, which kicks the deciding factor to "how advanced are the unlucky motherfuckers"
And again, if both start super established, the Flood probably win, Keyminds are fucked up