I don't have an answer for you, but I'm curious to know how the 6 do together in the 28 gallon. I bought a couple from my fish store a few weeks ago. I keep them in a 20 long. The lady at the fish store said I probably wouldn't want to keep more than 2 in a setup my size. They seem to do well enough, there was some chasing the first day but everyone has settled down now.
Fun little fish, but their small size in a relatively heavily planted tank means that I don't see them very much.
I feed mine bloodworms, and occasionally brine shrimp. I also have some frozen krill, though I haven't tried it yet. I have three planted tanks in addition to the puffer tank, so I have lots of snails. I've been picking the snails out of my tanks and dropping them in for the puffers. I haven't seen them eating them, but I do see empty shells around, so I assume they are hunting them. I only feed bloodworms a few times a week, so they will keep up hunting the snails.