I'd suggest fish oil rather than fish. A lot of cats seem to be sensitive to fish and will develop crystals and cystitis from it. Tuna and other fatty fish can also cause steatitis, a painful inflammation of fatty tissue throughout the body. Plus, cats are easily turned into "tuna junkies" and then won't eat their regular food, which really unbalances their diets and leads to big problems.
Flaxseed oil is another alternative source of Omega 3 fatty acids, which are all helpful for skin and coat problems.
Actually, too much brushing/combing can make dandruff worse. But most likely it's the dryness of the winter that's the biggest culprit--humidifier is a great idea.