I agree with just sizing up to a 75 gal. It would basically fit in any space you have set aside for a 55 just you will have a lot more options for small catfish. Truth be told there is no catfish that get to a descent size that I would recommend for a 55 unless it's a growout. If you went 75 gal I'd go with sun cats, a lima would live in a 75 for a few years, but would eventually need upgraded as I'm finding out with my lima. It's 16" after just 3 years with no signs of growth stopping. Stone cats would work in a 75 also, no heater required, and just keep them in a cool spot in your house.
BTW my favorite catfish I've ever kept was a Sorubimicthys planiceps. Very amazing growth rate. Second to none on coloration, and an attitude to match. Bury themselves on the bottom given the right substrate