I've advanced somewhat into the game (I think I'm past the first half, have done the Alien Base and 7-8 missions after it), and I'm still puzzeled about which aliens I can capture and under what conditions.
Here's what I've learned:
Some aliens are completly un-capturable/un-zappable, like the alien disk
Some need to be real low on life to be zapped (like Mutons must have less than 3 life)
for some aliens it seems (and I might be wrong) that given the same species and the same amount of life, sometimes the zap is succesfull, sometimes it's not (like the human looking alien, with 3 life, sometimes I zap it and it goes down, sometimes it doesn't work)
My question is then:
What are the unzappable alien species, and for those that are zappable, which ones need special conditions for zapping and what are those conditions?