Anyone of average build at height of ~ 6'7" (guessing) could probably play Nihilus.
as for Atton:
Oh yeah, Robert Downey jr.
Bao Dur: John Cena from WWE
Sion: Either Bill Goldberg, Keith Jardine, or Steve Austin
It'd be a stretch to get him to stop being such a goofball but
Bill Murray as Master Vrook