Hold on, I forgot about Newt. That would make 156 aliens max. Don't forget, there was a guy they talk about in the MedLab that the colonists removed a facehugger from, killing the guy in the process. And to comment on your first point Kain, I completly agree with you. Remember in ALIEN when everyone just wakes up and they're all eating at the table, or even at the Last Supper? Ridley purposely put a bunch of non-plot specific dialog in there that overlaps with each other to give it a realistic atmosphere.
Wow... why'd ya' do that?
Proof, I guess.