Currently, the human player models are 'segmented' - in models/players/human_base there is lower.md3, upper.md3, and head.md3. They move and animate independently. This allows, for example, a human to fire while running forwards, to reload while jumping, and to look around while doing all of these things.
The alien models, on the other hand, are non-segmented. The entire body has an attack animation, jump animation, gesture, and only one of these can occur at once. This looks a little silly when, for example, you hold down your gesture key as a dragoon and move around at the same time.
What I'm suggesting is a skilled modeller (or maybe an unskilled one, I'm not sure how hard this will be) takes on the project of separating the legs, head, and body, or whatever seems appropriate, of the alien models and thus allowing them to undergo several simultaneous animations in separate body parts.