lol every one has there own thoughts but my jd gets alone just fine with cons and i have a female jd and nver had problems. if there all around the same size it will be ok and all the jd will do is run the tank (show them whos boss)and then everthing will be fine....
the JD could eventually get a bit crammed in a 55g, however it could work counting none of the fish pair off and begin breeding. make sure you keep up with water changes however, because that could be a decent load on a 55g with a plec and once your dempsey grows some