I would bet the dempsey will rule the tank, but letting the grow up together does help them learn to tolerate each other somewhat. Your tank size is good, and if you provide plenty of cover, they should do okay.
You might want to get some think for the top part of your tank though, since these guys are going to be occupying the lower half. You might consider some Giant Danio's or some Rainbow fish. they both seem to be able to take the beating and keep on ticking! They will act as dithers, and this will also lessen the aggression on your other cichlids.