The Bughunters setting in d20 Modern may do the trick, if you can find a copy of the book. The setting is explicitly modeled after Starship Troopers, and the system is class-based. I've game mastered d20 Modern in a mix-and-match fashion for a couple of one-offs, taking elements from Bughunters and a couple of the other settings, and it worked well for fast-moving small-unit combat.
