Just felt like throwing my $0.02 worth of thought on this.
First I would say it depends on how you use the aliens in your game. The creatures from the Aliens, Xtro, and, Critters, films all can be used in a straight up horror game. The only sci-fi element that need to be touched on in a game is the question, where did they come from.
While coming from outer space, the creatures in Lifeforce, are really nothing more then a twist on various vampire myths.
While I cannot remember the title of the book off the top of my head, there was a vampire novel written a couple of years ago, where the vampires were the decendents of a race that was breed to fight in planetary wars.
IIRC, in Fred Saberhagen's novel, The Frankenstien Papers, we discover that after several failures, the doctor created his monster after discovering the gigantic corpse of a human near where a meteor crashed to earth.
So there is nothing wrong with using "known" monsters and giving them new backgrounds that have them either coming from space or the offspring of those first visitors.
Finally, everyone knows what a Grey Alien looks like. Thanks to movies, novels and TV, the image is one that has bombarded us for over a decade. However, back in the 30's Aliester Crowley claimed that while in North Africa he summonded up a demon named LAM. The curious matter is that Crowley's drawing of this demon, who appeared out of a bright light, looks exactly like a Grey. Think about it.