I wouldn't count on any puffer leaving your inverts alone, as that is their natural diet.
I do have a green spotted puffer in my reef right now because we didn't want to get rid of him when we tore down our FOWLR. So far, he has not bothered my shrimp, though I currently suspect him of having eaten my hermit crab (I prefer an innocent-until-proven-guilty approach). One of those would stay small enough. They're listed as a brackish fish but they start out freshwater and must slowly be acclimated to brackish or they will die young (4 years or so, vs ~16). Once acclimated to brackish they can be acclimated to full marine conditions. I do have to note that they are considered aggressive, but I have never had any problems with mine. He's a live-and-let-live sort.
