Considering that a Leopard Puffer is just about the smallest common saltwater puffer, a 30 would probably do. Although bigger would be better.
The thing about Tobies is that they are puffers. All puffers come down to personality. That personality can range anywhere from sweet and innocent to homicidal maniac. While different types of puffers may be more prone to a specific level of aggression, all of them are capable of anything along that range. In my previous tank I had a Valentini Puffer (Canthigaster valentini) that wouldn't bother snails and a Spotted Puffer (Canthigaster jactator) that tore Ricordeas apart and eventually nipped the tails off two Engineer Gobies, so there you go on potential range.
That said, Tobies tend towards the sweet and innocent side of the scale. Odds are that it'll be safe with your fish. It'll have the best odds of being reef/invert safe as just about any puffer. And considering that it is the smallest of the Tobies, it'll probably be safe with larger invertebrates. That said, be very careful on selection. Don't get one that looks like it is starving (hungry puffers tend to look at everything from the point-of-view of "is it edible?") and isn't attacking everything in sight. And consider a bigger tank, because the smaller the area the more likely a puffer is to get bored (bored puffers tend to look at everything from the point-of-view of "is it edible?").
