I finished a Silacoid for Deadzone plague as X-Com aliens project.
It's just a Barbie doll's shoulder joint and some green stuff. I really have no idea what it might represent in the actual game, because the only small based melee-only thing plague has is the plague hound - which is fast and die to an angry word. And Silacoid was the slowest alien in original X-Com and really tough. I guess it's just for the atmosphere.
I've been converting some Chryssalids too, and I used Mesaan Grunts from Hasslefree Miniatures for their head. Then I ended up with two torsos with only legs. Converting them to Spitters from X-Com apocalypse was pretty darn easy, and of course I had to do exactly that. The funnel head is green stuff and hands are from old plastic Games Workshop Vampire Count zombies. I probably won't do more Apocalypse aliens, because, well, they don't have that X-Com feel to them (though I did enjoy the game back in the days...)
As a side note... now I have all the aliens that Muton terror missions used for Deadzone. Whee...!