I'd say a lot of that would depend on the size of the puffer and the size of the food. It seems common to feed a younger/smaller puffer until their belly is slightly rounded daily with one day each week for fasting. As they age/grow the feedings would be reduced to like every other day, every two days, etc. Be careful though, as they'll happily overeat.
Just to mention about shrimp (or any other seafood that may apply) get shell on as that would be best for trimming the teeth.
I would think the puffer would have to be rather larger to crush a clam, but again that would depend on the size of the clam. If you open the clam the puffer would still be trimming his teeth in a way when scraping the meat from the clam. I know when it comes to feeding snails you feed the size of the puffers eye but that is a food that is more easily crushed in one bite.
I hope someone can give you more advice in terms of the size of food though you will probably have more of an idea once you see the puffer in person.
I haven't gotten my puffer yet so those are things I'll figure out in time too, I hope
Though, I'm getting a toby, so much smaller than a porcupine.
By the way, jealous! I'd love to have a tank of that size and the foxface is another of my favorites. They're beautiful fish!