Rd/Midas get up there in size i've seen up to 13". that being said, i would like to see that fish upgraded to a 75 gal or so. 4'x18" rather than 4'x12". it adds much needed width for the fish to turn around in.
they have also known to be very aggresive fish (which all depends on individual fish) some are not aggresive at all some are terrors to tankmates.
your tank also doesn't have enough room for a territorial or aggro fish like your rd and another fish to cohab if your Rd's picking on him. you'd need a 6'+ long tank for such cohabs of aggresive cichlids.
up till 10" or so he'll be fine in your tank. than look into buying a 125 gal or so. 6'x18" and you'll be able to cohab him with other larger cichlids. texas, oscars, etc. (not always but much more safely than in your 55 gal.)
you may have success keeping your Rd with a bottom feeder like a pleco or a catfish in your 55 gal. they are ussally ignored by cichlids.