There are many species of puffer that go into a 55 gallon, figure eight puffers, congos, etc. i would goto a puffer forum and do some more research on sizes and water requirements of different puffer species. Most need brackish water, some true freshwater species are the pea puffer, mbu puffer, and fhaka puffer, the only one of these compatible for your size tank is a pea puffer, of which you could have around 25 in that tank, but you would need a large supply of snails to feed that many lol and a lot of decorations.
Woops didnt realize you were looking at sw species, ill just leave this here lol in case youd want to do fw.