"The Sybiote only has to take over Plasticman's psyche."
I don't think that's going to happen...the symbiote would never be able to complete the bonding process. It could never completely envelop Plastic Man to overtake him. Plastic Man is to pliable and too maluable to get a hold of...Plastic Man would literary slip thru it's fingers...it couldn't be able to contain him to possess him...