I wouldnt put another puffer in there, PUffs are oooookay together as long as they are added at the same time. If you add other fish after a puff has established a tank as its own, it dont like company, and when the puffs get older they fight more often then when they are young, so dont get false hopes that all will be okay when things start out okay. If you are going to have more then jsut a puff in a tank, be sure to add the non puffs 1st, and if you have to add to a puff tank, reallly rearange the tank to make it look like a diff tank. let alone the puffers being nippy lil buggers, that is a tight squeeze for another fish to be in. I wouldnt recomend the addition
*edit* I have read on sites that as territorial as puffs are they require 5 gal a piece as a good rule of thumb.