Because of the narrow width of the 55 you can take much more advantage of space by going with a pair of smaller fish, something 6-8", rather then a foot long like a Severum. It allows for more decor in the tank, and with more decor and smaller fish, more exploring room for our pair. Basically better living conditions.
I'd do a pair of firemouths, nicaraguensis, Acaras, Rainbows, Festivum... the options are endless. Then I'd add a nice group of an appropriate type shoaling fish, and a group of an appropriate bottom feeder, and perhaps a pleco - species dependent on the cichlids chosen.
You end up with a nice and active setup while giving your monogamous pair the tank to live their lives out in peace.
This is how my 75 is set up (same length but deeper). A pair of Nicaraguensis, a shoal of 20 serpae tetra's, a shoal of 12 red tail rasbora's, a royal pleco, and a group of 6 syno. multipunctatus. The top, mid, and bottom levels of the tank are all taken care of, and the result is an active tank with all the fish happily doing their own thing. The nics get the whole tank, rather then being forced with another pair of cichlids (and they do use the whole tank when it's offered). I had a shoal of congos as well, but despite being ignored by all the other fish, they ended up being too skittish - sometimes a little experimentation is needed to create the perfect setup.