Well nothing that he might bite should be poisionus (lionfish,etc...) and any fish that can fit in his mouth and some that don't look like they ever could, WILL and he WILL eat them.
It depends on the size of your puffer. You could go with some hippo tangs, yellow tangs, Large tomato clowns, panther grouper (does get quite large tho)... Lions would be good... eels...
I'm afraid that your 90 gallon will be far to small to house the p.p. I'd say they need like 150 to 200 gallons . They also need to be alone or housed with much larger fish. They will eat any clean up crew, corals, smaller fish and probably bug equal sized fish to death.
hth
max