I noticed that, when two people are fighting against a single intruder in one room, one fighter usually moves closer and fights with the intruder hand-to-hand, while the other stands back and shoots them with something (lasers for the humans, little rock things for rockmen, etc). Is there any benefit to one or the other, and is there any way to use it to my advantage?
Shooters always fire at a random enemy. In 2 vs. 1, there's no difference, because there's only one enemy. In 3 vs. 2, or other situations, the shooter is supposed to distribute damage evenly by shooting randomly, but because of how random numbers work, one of the targets will end up with more damage than the other. (Random is not the same as evenly-distributed or nice.) This makes the other combat(s) in the room suddenly become much more lopsided after somebody dies.
The obvious difference is, of course, that only the melee fighter takes damage, so it would make sense to put the healthiest crew member (usually a rock man) into melee. With a little micro, you can swap out the melee fighter when he gets low. Other than that, there is no indication that there is a difference except graphics.