I like that defensive idea Psychobrat

As it stands the aliens do have uniquie abilities, which include wall walking, wall jumping, and eh... other stuff, that we may not want to give away just yet

As for the Alien only section, the problem with that is when it comes near the end of the game, the humans are claiming victory, all the alien eggs have been destroyed... but one alien is still alive, and he's hiding in a place the humans cant get to :/
its not fun, I've seen it happen many times in Gl... eh.. other mods.
we may not have Alien-only-vents, but I for one am putting vents in, which should give the same effect (exept knowledable humans may be will be able to persue).
But you can get the same sort of feel you're hinting at with just the wall walking...
you're in a big dark room, you think you see some movement on the ceiling... you look up but dont see anything....
little do you know there is an alien up there running in and out of cover, watching your every movement...
Fun Fun Fun
