Interesting. Let's see:
Energy beings: don't really fit the theme; anyway, they would be dealt with swiftly once you research lasers or plasma.
Grey aliens: no comment.
Insectoids: sort of everyone; they do work as a hive, with the Queen and all. More literally, Chrysalids.
Little green men: not really, although we have very big green men. I wouldn't mind seeing some though.
Nordic aliens: not present in the game. Perhaps these are our aliens' mysterious enemies?
Reptilians: well, Snakemen are this indeed, although the traditional Reptilians are way more humanoid and intelligent; they're supposed to be spies and negotiators.
Also, I'm fairly sure I've seen lookalikes of Floaters and Snakemen, and I think Silicoids, in some TV show on ufology. I might be wrong though, it's been years.