Tiger barbs are a strictly freshwater fish, hon. They can handle aquarium salt used for medicating freshwater tanks, but they're not a brackish fish.
For the cetnolucius hujeta, do you mean the freshwater barracuda/gar characin? That's what popped up when I did a search on the species name. Those aren't brackish either, and will probably eat anything smaller than 4 inches or so once fully grown--10 inch+ carnivorous fish, and it's recommended they get put in a 4 ft tank or longer, so you may need to upgrade again as they get older.
The flounder should do all right with a sandy bottom and sinking meaty pellets and a SG
of 1.005 or so. What I've read actually says that so long as conditions are met, they're fairly hardy.
For the brackish, since you already have a substrate dweller, maybe look at mollys, Celebes Rainbows, and bumblebee gobies, and orange chromides.