Not all gobies are brackish. Not good idea to mix puffers. First thing you need to do is decide specifically what fish you want and the type of water they require. This requires some research and your note taking skills. You would not want to mix freshwater fish with potentially a brackish or saltwater fish. Good simple research and note taking helps you decide what works and what does not.
a figure 8 with a knight goby would be a nice combo. Add a large short-finned molly to make the tank more diverse. All are light-brackish fish. Your tank is too small for a dragon goby. Dragon goby and mudskippers are brackish fish.