1 Severum can take over a 4 foot tank. I've seen it happen many times. Personally I think I'd do a pair of GT's or a pair of severums with some smaller cichlids perhaps (bolivians, guianacara, etc) and call it a day. To clarify, that's 1 pair of GT's and that's it for cichlids, or a pair of severums and maybe try some smaller geos like bolivians with the sevs.