agree with c-man, silver dollars like to be in schools, they also get big, like 6 inches, so with to sevs already in there it would tight to fit a nice school of silver dollars in.
Heros severus maxes out at 8"-10". Only the true mouthbrooding Heros (can't remember the rest of the name) gets to 12". I could be wrong but I seem to remember seeing that somewhere. So two 8-inch fish fill a 55-gallon? No dither fish or anything else? If you allow 15 gals. per severum, that's 30 gallons, add in a few dithers or a small bottom feeder to round things out I would think. Tank contains approximately 46 gals.
What about if they decide to breed? I wouldn't add anything else except the plec. Why add the bother of overcrowding.
Heros severum is the true mouthbreeding severum. The common severum usually seen in the shops is now believed to be Heros efasciatus. There are also some undescribed species in the shops at the moment like 'Rotkiel'