Yeah, not seeing too many 20mm sci-fi miniatures. My plan is to do some conversions of some Caesar Miniature sets. They have some WWII partisans and urban resisters, but that would only cover the non military characters. However they are coming out with a German Greatcoat set, so I might use those.
Thinking you may be better of going with 15mm now. Ground Zero Games has a bunch of sci-fi troopers at that scale, including some in powered armor. Eureka has quite a few 15mm miniature ranges it seems, even found some science fiction WWII Germans.