Add oil in a large skillet over medium high heat. Add shrimp and cook for about 1 to 2 minutes on one side. Season with salt, pepper, garlic powder, and cayenne (optional). Turn the shrimp and cook for another minute or so until the shrimp are pink. Add the enchilada sauce. Stir the shrimp to coat with the sauce and cook until the sauce is heated through. Stir in the pasta and coat with the sauce. Sprinkle with chopped onions and crumble white Mexican cheese over the top.