I wouldn't put a mbu with anything, IMO. A 3 foot puffer seems to be a monster that deserves it's own tank
Dwarf puffers are awesome, however they're so tiny you'll lose them in anything bigger than, say, a 30 gallon (depending on how many dwarfs you have). I would think you could keep 1-3 SAPs in the 55 with a few african peacock cichlids (not as aggressive as other africans, so less likely to beat on the SAP). I've heard many success stories about SAPs with a wide variety of fish, so I think it's trial and error, depending on the individual. And to the best of my knowledge, I'm pretty certain "leopard" puffers are the same as greenspot puffers.