When a xenomorph attacks, it's usually one of two things:
- They're in a position to do damage, so they roll damage (they have a marine pinned on the ground and they bite his face off).
- They're about to be in a position to do damage (leaping, rushing forward, popping out of a wall), so you say what the alien is doing then ask the marine what they do. This is usually (but not always) a "combat action" roll.
On 10+, they do it, taking fire as established, which means "established from the intersection of what the alien is doing and what the marine is doing." Sometimes this means rolling damage anyway, or reduced damage, but that's up to how brutal the situation is.
On a 7-9, I often do reduced damage or stress, or sometimes destroy gear. "It bites into your helmet instead of your face, tearing it into metal and kevlar shreds. What do you do?"
Enemy vs. group, yep, use the Parity rules. You can rule that a single drone counts as a fireteam or squad if you want to be really nasty. But for "hordes of aliens" games, treat each drone as a soldier.
No worries on the questions! I like rules questions.
Good luck on your first game. :)