Midas/RD are great fish. If you plan on keeping it in the 55 gal then tankmates are probably out mine killed almost everything I put in with it in a 125. Especially if it is already aggressive.
Red Devils has a pointier nose or lips, the Midas has a thicker nose or lips. I love my midas he is mean, but he is a cichlid. Mine grew from 2in. to about 12in. (not counting the tail) over a 2yr. period in my 84gal. I have kept mine over crowded his whole life, and never lost a fish to him. Hes an adult now, so over the last 2 months Ive had to get rid of my two 12in. Flower Horns (Snake Skin & Blue Dragon). Now the only fish his size in my tank is my Peacock CICHLID (they were named peacock "bass" to get people to come fish them)