Alot of people put them in fresh water. Along with green spot puffers and mono's and archers. But they are all brackish fish. People do it and you can to but i would not. Just letting you know the facts.
Not aggressive at all, except to other dragon gobies during feeding time. These guys are basically blind and are filter feeders so they can require special care to ensure they get their share of food, especially when in with other fish.
Plan on brackish for long term health. Either target feed when smaller or put in with non aggressive feeders. Unless there is a big enough source of small food it will not compete with fast eaters and starve.
I have mine with a ropefish in a 75 gallon, (as of now i am trying to get a brackish setup going but i can't find a cheap tank) the only thing he ever didn't get along with was my dwarf frogs at feeding time, they would bite at him so he would almost fit them in his mouth, usually happened once a week