A pair of Severums need a minimum of 55 gal, so having them and a fire mouth is definitely doable. The feather fin catfish would also be fine, but I'm not sure about the silver dollars, just because the Severums and fire mouth do get pretty big (up to 10 inches) and if they don't feel like they have enough room they will get territorial. I'm not sure though, you could get away with having a couple of silvers while the sevs are juvenile but not sure how they'll go when they get bigger!