Tropical and brackish are not mutually exclusive. Tropical means
warm, and brackish means somewhat salty. Many brackish fish ARE
tropical, and many tropical fish are brackish.
You can keep tropical brackish fish with other tropical brackish
fish, if they have the same temperment and will not harm or eat
eachother. You can keep some brackish fish with some freshwater
tropicals, and similarly you can keep some brackish fish with some
marine tropicals. There are no wide open set of principles for
brackish fish compatability beyond those that I have just
illustrated.