Combine the vinegar, sugar, and ¼ tsp chili powder (or more) in a small saucepan. Bring to a boil. Remove the pan from the heat. Stir in the onion and the radishes. Let stand, uncovered, for 10 minutes or until cooled.
Rinse the fish. Pat dry. Gently toss the pieces with ¼ cup of lime juice, ½ tsp (or more) chili powder, and the minced garlic. Marinate for 10 minutes at room temperature.
Gently toss the mango and avocado (if using) in a small bowl with the remaining ¼ tsp of chili powder (or more) and 2 Tbsp each of cilantro and lime juice.
Preheat a lightly oiled grill rack, or lightly oiled stove top fry pan. Remove fish from marinade, and grill/cook, turning once, about 2 to 3 minutes per side. Transfer to a plate, cover, and keep warm
Grill tortillas until slightly crisp. Form them into a U shape.
Fill tortillas with fish, pickled vegetables, mango mixture, cilantro, and salsa verde (if using). This would be a time to add the guacamole if using.
Cole Slaw is a complementary accompaniment. Could even be put into the taco.