Prep: 20 minutes; Cook: 15 minutes; Marinate: 10 minutes.
Combine the red wine vinegar, sugar, and 1/4 teaspoon chili powder in a small saucepan; bring to a boil. Remove the pan from heat, and stir in the red onion and radishes. Let stand, uncovered, 10 minutes or until cooled.
Rinse fish; pat dry. Gently toss with 1/4 cup lime juice, 1/2 teaspoon chili powder, and the garlic in shallow glass dish. Marinate for 10 minutes at room temperature.
Gently toss mango and avocado in small bowl with remaining 1/4 teaspoon chili powder and 2 tablespoons each cilantro and remaining lime juice.
Preheat a lightly oiled grill rack or grill pan. Remove fish from marinade, and grill, turning once, 2–3 minutes per side or until cooked through and golden brown. Transfer to a plate; cover and keep warm. Grill tortillas lightly until pale golden and slightly crisp. Transfer to 4 serving plates, folding tortillas to form U-shape.
Fill tortillas with fish, pickled vegetables, avocado mixture, remaining 2 tablespoons cilantro, and salsa verde, if desired; serve.