Combine fish, lime juice, orange juice, oil, and serrano chile in non-reactive glass or ceramic bowl. Cover with plastic wrap and refrigerate for 90 minutes.
Mix in mangoes and green onions, cover, and refrigerate for 10 minutes more. Gently fold in diced tomatoes and 1⁄2 cup of the cilantro. Season with salt and pepper and gently fold again. Garnish with remaining cilantro.
Divide ceviche evenly on tostada shells and serve immediately.