I would go for one of them, since both Red Heads and SD's do best (and i like them best) in groups (I would probably take 7 or 9).
I think 15 of these fish in a 120 gallon would be a bit much. On the other side, something like 3 red heads and 5/7 silver dollars should be possible i think. Geo's are mostly near the bottom, while SD's will go literally everywhere in the tank. So they probably won't bother each other too much.
I had Hoplarchus Psittacus (true parrot) with Geo's together in the past. The parrots were real bullies sometimes., buth with a good scaping i never had any problems.