Just my $.02...
I've come across a LOT of contradictory info about the dwarf puffers, that they're brackish, no freshwater, or best kept singly, etc. My experience says they're freshwater, and they in fact prefer to school.
I've got five of them, and they were FAR too nasty too keep with my smaller, peaceful community tank. Fin nipping on the cories and badis, but no deaths or killing. They were moved temporarily into a small grow-out tank with baby bichirs (sens), where they lived for about a month. No fin nipping, everybody got along great.
They've since been moved to a 55, along with the bichirs and some other misc fish, and they couldn't be happier or healthier. They tend to travel the tank in a pack, sometimes a straight line of them (very follow-the-leader), and don't separate for long. They're amazing little monsters, VERY inquisitive, fearless, but agile enough to stay out of harms way. No one seems to want to mess with them, and they don't seem to care to bother any of the bigger (significantly bigger) fish in the tank.