Hmm, this sounds familiar :)

I whiled away many an hour with my mates on alien swarm - possibly one of the best UT2004 mods ever released.

Pretty much identical to what's being proposed here, except for the fact it obviously has nothing to do with the aliens franchise as they're not licensed... except for the aliens, eggs, pulse rifles, flamethrowers, sentry guns & marines of course :P

Break out that copy of UT2004 and get it from the link below - be warned, it's fucking *hard* - but addictively so :)

http://www.blackcatgames.com/alienswarm.html