So is the new rule for big figs (Starro, Foom, etc) is that they can not be the target of Possession anymore? They reason I ask is becasue the Anti-Monitor can't. It says in his rules, but is he the only one that can't be and the others can?

They haven't announced any new errata yet, but I wouldn't be surprised that when they do, other colossals will not be able to be the target of possession.