I agree with you on that their main diet is invertebrates in the wild. So generally you should not mix them together. However, I think whether or not your puffer will eat your invertebrates depends upon what species you refer to. I've had both large puffers (including dogface, striped, immaculate, and map) and sharpnose puffers (valentini, bluedot, bennetti, hawaiian), and I found out that all of tobies were more than happy to consume all of my invertebrates. On the contrary, all of my big puffers did not even bother them at all. I think this was because I trained them to eat only frozen food when they were really young. I know these are just a few exceptions, but from my experience puffers are very intelligent and they can learn not to eat crustaceans.