I would say if it is a 20 gal long yes. They might chase each other now and then but if they have enough room for the Cockatoo to have his own territory it should be OK.
i've had them with rams before with no problems, so long as they have plenty of room, caves and hiding places they should be fine.
as with all cichlids it's somewhat risky but i think it's certainly worth trying, just have a plan B in case they do fight