The Dragon Goby is definitely going to need more than a 5 or 10g tank. They are said to get 2 feet long in the wild, but probably wouldn't get quite that large in an aquarium. They probably hit around 12" though, so you'll want atleast a 3ft tank for it, if not bigger. Skip the 10g and shoot for something in the 40g range for now.
Here are some pics I found of a 2ft DG that lived in an aquarium and was actually kept in freshwater (though I wouldn't recommend it):
http://www.practicalfishkeeping.co.u...read.php?t=317. That is one huge fish.
To make a tank brackish, I believe you need marine salt.
