There is no puffer that is truly reef safe. Some are better than others. With the tobies (sharp nose puffers) some individuals are better than others. My Papuan Toby will ignore mushroom polyps and palys, but was so focused on trying to eat a long tentacled anemone that he killed it over night. Stinging cells don't stop him, only poison works, and even then I have seen him take a bite of my big, very toxic palys when I forget to feed him twice a day.
Nasty taste also works. He only ever took one bite out of the Kenya tree coral before deciding it wasn't edible. I don't dare try any small polyp hard corals, and I'm leery even of LPSs.