There isn't really a single "wet pet", non cichlid, that is active and can fit in a 55 for life. You might have to switch it up. Maybe go the opposite route with a lot of active smaller fish. Some smaller barbs, corys, and a crayfish. If you still want to do the wet pet thing, get some dwarf puffers.
Man I absolutely love the Colomesus asellus. The only reason I don't push them like a dealer is I have not had success keeping them. I'll have to hit you up for tips if I try them again.
^ Dwarf puffers are = Carinotetraodon travancoricus, the ones you are stalking about are commonly known as the South American Puffer... great puffers if you can find a group of them and dont mind a little teeth trimming...
Ok OP, my first suggestion, although a cichlid, would be a breeding pair of discus. They will be beautiful in the tank and since you're going bare bottom, maintenance for them should be pretty easy.
Another suggestion, a bottom dweller, would be 6-8 pictus cats. In a group such as this they should be pretty active and fun to watch.
What I've read so far is that they like their planted tanks. From experience keeping fish that enjoy plants and foliage in a bare tank stresses them out and makes them hide. Having a fish that hides is a no go for me. I've come to love the personality and just "in your face nature of the oscars, channel cats, goldfish and even some of the salt water species I've kept over the few years. I guess what I'm getting at is I want something that is out most of the time.
A Green Spotted Puffer would be good, the brackish set up isn't hard to maintain. And you have a saltwater tank so you obviously wouldn't have any trouble with it.
You're from Schoharie? I'm an upstate New Yorker too. Eddie's has decent fish, Davey Jones Locker is cool too.