Its going to depend on his temperment. I have 2 RD's 1 is in a tank with a Jack Dempsey, Pike Cichlid, and Zungaro Cat. The other is in a tank with a Jag, Jack Dempsey Green Terror and Myan Cichlid, along with some dither fish. I have a few problems with the first tank as far as aggression goes nothing serious though, but they will be moving to a 150 gal in a cpl ofweeks.
You could try any aggressive cichlid but make sure you have plenty hiding places rock caves rootwood etc to break up the tank and to give the fish a place to retreat to and/or establish a territory of their own. I would add some medium size fast fish as dither fish to hopefully decrease the aggression. I have tinfoil,albino, and clown barbs, in my more aggressive tank. I am going to buy some bala sharks for the new one.
Good luck