I might got so used to this mission that I failed to see more aliens spawn.
I always follow this order:
* enter ship (you stand in the corner opposite the people you have to rescue)
* don't move and kill the small alien (which spawns the other aliens)
* kill the alien in the corner to the right and MOVE to that corner
* if you've killed it, a medic alien spawns in the corner you entered the ship
* kill the medic and MOVE back to the corner where you entered
* if you've killed it, a technician spawns in the corner where you've killed the 2nd alien
* kill the technician and move to that corner
* the general spawns
* loot the item, ignore the alien near the people, give item to people, run out
As long as you stay in the corners no alien will aggro you.