There are big nasty Greys from the old Harbinger range, oop but you can find them n ebay often enough. The bio robot is from Copplestone and goes quite nicely with them.
Spice lice are from Irregular Miniatures and are cheap and plentiful.
Wargames Supply Dump makes a lovely Cyclopean.
D&D plastic grells are quite 'psychic' looking.
Ropers make good oldschool aliens (Otherworld and D&D)
Black Cat Bases' Greys are heavily armed.
Spriggan Miniatures' Spugs are nice.
There's lots out there. What greys are you using. My 'little' Greys are from Tengu: