First off if you dont plan on getting a 75 gallon within the next year leave the RD out of the 55 gallon. You main issue will be once the RD gets some size to him the 55gallon will be to cramped and he will more then likely kill your convicts. Also convicts like to breed so if you have a pair in there it definately will become a problem with the RD...GoodLuck...
Of course you can keep the RD with Con's. . . even once he/she gets adult size. . . of course on that same note you can have fancy guppies and goldfish in there too. . . if you're lookin' for the underwater version of the UFC. . . otherwise a 75 or even larger would be advisable. . .