Your dat will probably grow slower. Dont worry about what the LFS said. They have not been doing their research. They might flare up and get upset with each other but they are not going to go on a killing spree like some other cichlids. Attitude can change from fish to fish though. If they are orinos they might have a little bit more of a temper.
I have tons of cichla to together in one tank, and I have had another batch grow out together. They just fight for dominance and then they are done, unless they have insufficient tank space.