Fig 8 sadly won’t go well with inverts (puffers are puffers), but isn’t as mean to fish as some other puffers (still very aggressive). That said, they are messy and I wouldn’t really keep it with more than a single bb goby in a 15 gallon (60 L) as that is its minimum recommended tank size. No guarantees that it won’t kill the goby though. They need very clean water and salinity of about 1.005 (a reef tank is 1.025 for reference). Almost all puffers need to be fed hard shelled foods like shrimp and snails to keep their constantly growing beak trimmed. They also love bloodworms (but don’t feed them bloodworms too much). Puffers are highly intelligent and learn to recognize their owner very quickly. Fig 8 is about 3 inches fully grown.
The hermit crabs are standard blue leg hermit crabs that you may find at a petco. They do well in 1.015-1.028. Very easy to take care of, only hard part is making sure they have shells.
I don’t know too much about brackish or freshwater gobies so I will stay out of it there.