I would also suggest a 75 gal
Your snails and hermit crab may get eaten unless like you said you keep it well fed. The puffer may nip at the goby though depends on the sizes of the two.
The tang will be fine. Becareful both tangs and puffers are prone to ich. So keep the water at optimal conditions.
Cowfish are awesome fish. IMO
have more personality than the puffer. However if they are stressed they can exude a toxin that can wipe out the whole tank. I have one and he is with damsels, but he holds his own and doesn't take any crap from any fish. I have heard of people having problems keeping cowfish but my experience has been nothing but ease and enjoyment. I have kept my cowfish with puffers as well so if you want get both (if you have a big enough tank).
hope this helps