You have to feed them live food, they will rarely take flake or pellets.
I feed mine grindal worms, blackworms, daphnia, mozzie larvae and I think they eat the snails in the tank (have never seen them do this though). Mine will not eat frozen bloodworms or frozen anything for that matter.
They need approx 20l per fish - sounds extravagant for such a wee beastie, but they need it along with plenty of 'cover' such as plants so that they can stay out of each others' line of sight. Any other fish in one's view is just asking for a bite, no matter what the sex. Try and get one male to two or three females or there will be a war of epic proportions when they get a bit bigger if there are two males, usually culminating in the death of one.
I have ottocinclis in the tank with mine, they act like dead leaves most of the time and the puffers seem to tolerate them in the tank, anything else would probably get harassed and nipped to death. They are feisty wee monsters, I don't know how one of mine survives, when you feed them he starts heading towards a worm until he sees another fish out of the corner of his eye so he has to go chase them off, this done he looks for more food until he sees another fish out of the corner of his eye and... you get the picture - why he is not as skinny as a rake and half dead I do not know.
As disgustipated says they are full of character and cruise around with seemingless effort using their pectoral fins and bending their caudal fins around like a rudder to steer themselves, this they usually keep clamped so they look like little fat torpedoes and they only flare them when they are doing the 'don't mess with me' dance around each other in circles.
Don't buy skinny specimens, they should be plump no matter what the size, and ask the seller what they feed them, if it is not live food but they say they have seen them eat flake I would be a bit skeptical and they look a too thin I would give them a miss.