Sorry forgot to get to this post yesterday. Don't be mislead, thats not "About it" that can go in there.
In my experiences (though i'm sure it varies cichlid to cichlid, tank to tank, etc) severums have been quite mellow and not messed with anyone.
55gal is a nice sized tank, but its pretty narrow so there isn't a whole lot of swimming space, but there are a few choices. One thing that was suggested was firemouths. Could do some acaras, or what about another Severum? Possibly a dwarf pike. If it were me i'd go towards a pair of Severum, possibly to attempt to breed them.
I'd look for anything in the mid size range (Larger then dwarfs but smaller then tank busters like JDs, red devils, etc) and simi-aggressive.