puffers are in my opinion one of the best fishes in the trade as pets, they've got the greatest personalities. if that puffer you saw is still a juvenile, i doubt you'll have any problems with him eating other tank mates. many puffers are actually too docile for their own good in my opinion. while they have the capabilities to chump up anyone in their way, often they are easily intimidated and stressed by other tank mates, even if non aggressive. mine cringes and do flips if the false perc swims to close to him sometimes, even though the clown never intends to bother him. its rare that they go after other fishes...they prefer sessile things and if they have a taste for fish its usually frozen or dead ones. in the natural environment they prey on sessile, slow moving crustaceans and other hard shelled inverts, not fishes. there was one time at night when my tank was already dimmed and the inhabitants already asleep. i had stuck my hand in to pick up a seaweed clip that was near my sound asleep clown, when my puffer woke up to swim near my hand as he always does expecting food, he took a small chunk out of the dorsal fin of the motionless clown near my hand thinking i was feeding him a dead fish. when the clown was rudely awakened and flinched the puffer scurried to the other end of the tank as fast as i'd ever seen it swim...that said i've never seen a puffer chase after a live fish, so i think your fishes are safer from being eaten than the puffer from being stressed by them. your hermits are another story. however there are precautions you can take even if the puffer is already big. before finally introducing him to the tank put your hermits near their hiding spot. like irenicus and small fish said they are smart enough to recognize a predator and when to best come out. they would likely do their roaming and feeding at night when puff is sleeping. i had my big red hermit for a long time and my puffer has even gave up bother it. he knows that when he gets too close the hermit is far in its shell already, and as stubborn as he is he has gave up on the hermit. i think you might be safe...but its good to be precautious anyways.
