Combine pineapple, sweet red pepper, scallions, lime juice and hot pepper flakes. Set aside.
Place flour and cornmeal in separate shallow bowls. Whisk mayonnaise and Old Bay seasoning in a small bowl.
Dip one fillet in flour; shake off excess. Brush both sides of fillet with mayonnaise, then dip in cornmeal, shaking off excess. Place on waxed-paper-lined baking sheet; repeat with remaining fillets. Can be made up to 3 hours ahead, covered and refrigerated.
Heat large nonstick skillet over medium-high heat. Add oil, then butter. Add fillets and cook for 3 to 5 minutes, or until golden. Turn and cook another 5 minutes. Serve with pineapple salsa.