IIRC Aliens will target wounded troops, and maybe troops with lower health, often the fresh meat brought in to replace last missions dead.
Aliens will run out of ammo and reload - pretty sure I have seen this evident from plasma rifle fire - after a few (3?) bursts of auto fire, there are 2 rounds left to be fired before reloading. Eventually, I have seen plasma rifle toting Taman and Sheevar drop thier rifle and rush with a plasma blade or grenades, should a combat go on for long enough (often on maps making closing in impractical).