First off, I've never seen the turrets in AvP - I didn't model and skin these from any concept except the ones in my head.
Secondly, FOX does not have intellectual property rights on an oval health bar nor turrets. It has to be something distinguishable. They produced a movie about aliens. Since that time 100's of movies have been made containing aliens in which FOX does not own any property.
What FOX does own and protect is the Aliens brand name and anything that tries to infringe or compete with it.
Aliens for Quake1 was trying to recreate the Aliens universe using the Giger aliens which are very specific. FOX had a commercial product and did not wish to have any brand tainting, I don't blame them. Since then though there have been tons of mods and games featuring aliens. Even Invasion mod for Q3 had the actually Giger alien in it and posed no threat.
You can download the Alien here :
http://www.planetquake.com/polycount/info/quake3/alien3/alien3.shtmlAnyways, Tremulous doesn't feature any aliens from Aliens , it doesn't even feature any likenesses regarding guns, vehicles, huds, etc.