In a pie pan or dish combine oil, 2 tablespoons lime juice, chile powder, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Toss fish in mixture and then chill for 15 minutes.
Meanwhile make relish by combining relish ingredients in small bowl (cucumber, onion, oj, 2 tablespoons lime juice, sugar, remaining salt (1/2 t) and remaining pepper (1/4 t). Set aside.
For the dressing combine mayonnaise, sour cream and remaining lime juice (2 T) and refrigerate until needed.
Prepare grill and spray with Pam and heat to medium high. Grill fish 2-5 minutes per side or until done. Remove from grill and let cool slightly before dividing fish pieces in half while warming tortillas on grill (less than 30 seconds per side) -- wrap in foil to keep warm.
Stir serrano chile into relish.
To make tacos: spoon 2 T relish into each tortilla, flake fish piece onto the tortilla and then top with salsa and drizzle with the sauce. Fold tortilla (taco) in half and serve.