I have had two dwarf puffers in my tank for about a year and a half. They're very cool little fish. With them are two medium silver dollars, two raphaels, a royal pleco and a ripsaw catfish. Everybody's happy and everybody leaves each other alone, though I just added the ripsaw a few weeks ago.
Anyway, I feed my puffers frozen brine shrimp and bloodworms. I have had a hell of a time getting them to eat anything crunchy to wear their beaks down. I have heard they like frozen clams, though I haven't had any luck yet.
Puffers are pretty aggressive things, and even little dwarf puffers will stand up to a fish many, many times its size. I have heard they can and will pick on other fish. Most of my fish could, if they wanted to, eat them, but everybody leaves the puffers alone.
) Most other kinds of fish I like to keep in pairs but again, it depends on the species of the fisn.