All those fish you mentioned can handle brackish water conditions but I would recommend limiting your stocks for now and save up until you can afford a larger tank for the dragon goby.
When I had my Violet goby, I had to actually hand-feed him with a pippet. He was so funny though! He would get so excited when I started feeding him.
He sure showed me that showed me that fish can be intelligent!
Has anybody else had this with their violet goby?