The best fish tacos I have ever had always use a cilantro-lime aioli. Pico de gallo can be good with white fish. Black beans cooked with corn is good for baja style. Use corn tortillas (toast on your grill or gas oven on both sides).
For your fish - I have seen everything from tilapia to flounder to red drum to salmon. It really depends on what you are going for. I love ceviche tacos made with mango-lime, cilantro, red snapper ceviche. Fresh fish is best. You can't do the snapper ceviche unless you got it live, sorry. For frozen, flounder is probably the best. Season with 50/50 taco seasoning and cajun blackening and oil both sides and grill or pan fry in cast iron. I make my own taco seasoning - the store stuff is all fillers and no spices.