Well, the Website about this Halflife Mod did somehow not work with my browser (But I couldn't play it anyways, as the only PC I have is VEEERY slow - P75, and Halflife is not available for other OSes).
Anyways about "Marines vs. Aliens" there are two Mods (all in 1st Person View, though) for Quake 2, both ported to AmigaOS:
Gloom - Aliens in the "Aliens Movies" sense
Roughnecks - Aliens in the "Starship Troopers" sense
Roughnecks is only playing AGAINST the Aliens, though, not playing the Aliens. But in Gloom you can also play as Aliens (Alien-Team vs. Human-Team, with various Alien/Soldier types with certain Abilities and such - class-based Mod)
Steffen