It would be a combined scenario with some home homebrew elements. Alot of it would depend on the setting
I usually mix other aliens in.
these things can be alot of fun.
by crossover I mean three or more groups
they are most fun with a mixed group of veteran players.
Lets see ive done...
a tyranid and xenomorph one
really want to do a yuatja one
the essence is to have multiple groups of aliens and/or human players
usually it takes more prep but can be surprisingly fun