They're not quite as bad as the rest of these guys seem to think, in reality it's just going to depend on the fish, they vary.
If you wanted to just keep one in your 55 with a single Tomatoe Clown you'd probably be fine they just need to be watched at first.
I've currently got 2 in my 90 gallon SW tank, for the moment they're in there alone but at one point there was an Lawnmower blenny in there with them which they left alone, though the Blenny died for unknown reasons(no bite marks though), currently I have another blenny in QT waiting to be added and plan on adding a Wrasse and Tomatoe Clown. They are nippy, though I attribute it to curiousity more than aggression, I've got several soft and LPS corals in my tank almost all of which they've nipped when I first added them but have since left alone realizing they're not food and the corals are doing fine.
There're some Puffer specific forums call simply "The Puffer Forums" online that may have a bit more information, in short others have had luck keeping them with other fish including Tomatoe Clowns and Wrasses. The biggest concern is going to be with inverts, small snails and crabs may be eaten but honestly GSPs are just to small to chomp through the shells of most marine snails or get at hermits, I've had several hermits in my tank for the few months it's been up and I've only lost one, the others are too quick, they retreat into thier shells which are way to thick for small puffers to bite through.
Edit: Oh also if anyone is curious I started my two puffers out in a freshwater tank the way they came from the fish store, transitioned them to a brackish for several months, and then took them to full marine over a few weeks going from 1.008 to 1.024 salinity no problems with live rock added around 1.020.
