What restrictions are there for putting FCs on any/all of the colossal figs? I seem to recall there were restrictions set out but can't remember them for the life of me.

I believe that as long as they are being played "as part of a force", and they meet the pre-reqs for the feat, that feats can be applied.

As long as the prereqs can be met, it is fine to assign feats.

That said, there is errata to prevent Shellhead and LazPi from being assigned to them. Additionally, Spectre and Galactus cannot be assigned feats. (Probably Starro and Foom can' either, but I'm not 100% sure on that.)