Combine garlic, cumin, 1 tablespoon chili powder, and 3/4 teaspoon salt in a small bowl. Stir in lime juice and vinegar. Whisk in sour cream and mayonnaise. Slowly drizzle in 1/4 cup olive oil, whisking constantly. Stir in bell pepper, carrot, and red onion. Cover and chill at least 30 minutes.
Brush fish with remaining 2 tablespoons olive oil, and rub with remaining 2 tablespoons chili powder and kosher salt to taste. Grill fish over medium-high heat (350° to 400°) for 3 to 4 minutes on each side or until fish flakes easily when tested with a fork.
While fish cooks, wrap tortillas in damp paper towels; microwave on high 1 minute or until pliable. Using a slotted spoon, transfer bell pepper mixture from the dressing to a large bowl. Stir in cilantro and serrano chiles, if desired. Add cabbage, and toss thoroughly, adding additional dressing if desired.
Spread tortillas evenly with guacamole, leaving a 3-inch border at the bottom. Top evenly with fish, slaw, cheese, and pepitas. Fold the bottom edge up, and roll from one side to the other, encasing the filling.