To answer the non-predator/xenomorph  aliens question I'm honestly not sure if there were any documented instances.... I know in the comics/books there are these things called rhydons, or something like that. And they are basically some sort of mix between a cow and something else (and about as smart). Also I'm personally of the opinion that the Avatar vers and the Aliens/Pred verse are one and the same just with Avitar taking place well before the Alines time line.