I think they could look good (a bit RT even), but I'd expect to see a bit more than a suit of armour and a modern gun; I think they'd need some cables, maybe lenses in the helmets, and possibly a few other small details to "sci-fi" the armour a bit.
As for guns... Hmm. Heresy Miniatures do some that might work in size? Failing that, I think Wargames Factory do some sci-fi kits you could rob guns from?