i had a dragon goby for several months, who only died due to a heater malfunction i dident notice. mine was in FULL salt water, and ate like a pig and was growing quite nicely. i had done alot of reading about them, and found that they are occasionally found living in shallow areas of the ocean as well.
mine was reluctant to eat at first, but finally starting taking tiny sinking pellets, bloodworms and the like. despite their big mouths, they cant eat as big of food as you may think. as long as it has a cave/tube to hide in and clean water, he should come around to eating soon enough. just give him a few days to adjust like yo would any other fish.
as for tankmates, make sure their as passive as possible, since the dragon has extremely tiny eyes and it can be difficult for them to find their food, and can easily be outcompeted.