My porcupine puffer has never so much as glanced at my chocolate chip seastar. He does, however, munch on the snails in my tank from time to time (but they're breeding so the supply stays fairly steady).
The general rule I've found with mine is he will eat whatever fits in his mouth. Most things that can't fit, he will try to "sample" once or twice and then ignore afterwards.
That said, I have many, many snails, hermits, a feather duster and the seastar in the tank with him and with the exception of the smallest snails getting picked off now and then, he bothers nothing else.
The level of their aggression varies pretty broadly, though. I've seen some porkies that just lie around all day and some that constantly swim, biting at the glass and attacking everything. The best thing to do is monitor the puffer closely before adding any inverts.