I make fish tacos with leftover grilled fish. Flake the fish, wrap it in foil, and heat it in a low oven. It doesn't have to be super-hot for the tacos, which prevents the fish from getting overcooked, which is the biggest challenge in using leftover fish.
I prefer corn tortillas to flour but you could use either. For condiments, I usually make a creamy cole slaw with mayonnaise, Chipolte Tabasco, garlic to use as a condiment. I also buy fresh salsa and add avocado, mango to it.