In my experience firemouths do better in groups of five or more (especially in a tank as small as a 55gal). In smaller groups the smallest firemouth often gets harrassed so badly it either gets sick and dies or lives a life not worth living. In a 55 gallon I would personally get a good group of 7-8 firemouths (juveniles) and let them grow out. They are truly fascinating creatures and there complex social interactions are truly a joy to observe.
Silver dollars are alright, and I've kept them a number of times, but in my opinion they get a bit boring after awhile.
My suggested stocking school of firemouths, a few s. nigreventis or one s. decorus, and perhaps a group of 3-4 corys
Carl
