right, you can't have anything else in the tank. Be careful what specices you buy and do your research, some species are brackish and others grow huge.
They must be fed meaty foods. Bloodworm, cockle, snails. Snails are especially good as they keep their teeth down.
28 gallons. Well...you hhave a few options there! Red eyes, SAPs, these are just two smaller species. you could also have a few dwarfs in there.
Good choice with choosing a puffer!