Severum's only get to about 8". You could probably keep about 4 in a 55. You could probably keep 1 or 2 jack dempsey's, green terrrors, texas, although most people would tell you that you can only keep 1. You could keep a lot of convicts or firemouths because they only get about 6".
I have 3 severums (1-gold, 2-turquiose). They are awsome. The tips of their dorsal and anal fins get really long (as long as they don't bite each others fins). If you decide to get a gold one look for one with lots of red dots on them. That's a better quality one. For some reason a lot of them are just gold in color and don't have the red markings on them any more. Google it and check out the difference.