Dice shrimp and place in a mixing bowl. Squeeze lime juice over shrimp until they are completely covered, about 4 limes. Stir in the tomatoes, onion, cucumber, serrano peppers, and salt and pepper. Cover, and refrigerate 1 hour.
When ready to serve, adjust seasoning with salt and pepper as needed. Serve on top of tostada shells with a splash of hot sauce, if desired.
