^ I like that mix cody... Most adult sevs that I have seen in person top out at about 9" TL (they are deep bodied and thick at that size), but I have heard of Sevs getting to be as big as 12" - so I can totally see why some people feel that a 55g is the minimum...
Having said that I prefer having tankmates with my fish as the situations permit. My severums were very accomodating to tankmates - but they did eat some tetras... But sinisterkisses (another member here) had a Severum that would not tolerate any tankmates.
At the end of the day a big part of the success or failure will depend on the individual fish in question.
well right now it has a 7 inch oscar for a tankmate that I just convinced her to get rid of. We're taking him to a pet store with a huge oscar tank. The severum is not the dominant of the two, so if another cichlid would fit i'm sure we would try it. Might look into the festivum I saw some yesterday at an lfs.
That's a good decision... Although I believe that Oscars make excellent tankmates for Severums a 55g tank is not an appropriate size for such a combination... Aggression may not be a problem, but the limited space and heavy bio-load (let's face it O's are pigs) could easily lead to less than ideal water conditions and/or a maintenance regimen that saps the fun out of the hobby and makes it feel a lot like work.
If your Sev will accept them Bolivian Rams would be an excellent choice to try adding to the tank... If it were my tank I would try the Bolivians, or maybe a Keyhole or Festivum, a Steinie as cody has done, or maybe an L. thayeri or two... (but not all of the above

).