How about some invertebrates? I feel like, with the options listed so far, you've got bipedal tetrapods pretty well covered. I personally like the idea of an insectoid (or at least arthropod) alien in the mix, and a gastropod would be pretty entertaining as well, plus either would definitely stand out as pretty alien next to the various other more-or-less humanoids in the squad.
From plastics, an arthropod could be made using tyranid bits, but that might be a little too recognizable, so I might instead suggest Mantic again. The Z'zor or Nameless from Dreadball would be a good starting point.