Shame on your for buying a fish you knew nothing about! heh
Moving on, from what little I know about freshwater puffers, I know they prefer a brackish aquarium, but just recently I've seen a couple in my LFS's planted setup. I also know they are pretty picky eaters, as you have experienced. Not sure what else they might eat, you could try getting them to eat some frozen foods (use a turkey baster and squirt towards the puffers and away from the rest of the tank population). Not sure on size issues though, as all the FW Puffers I've seen have been the sixe of guppies.