Add serrano chile peppers, lime juice, olive oil and water to a blender. Add garlic, oregano, cumin, cayenne and salt to blender. Blend (puree) until almost smooth. Toss marinade mixture with shrimp in a large bowl and let marinate 30 minutes.
Halve and pit the avocados. Scoop out the flesh into a bowl and mash. Mix in the white parts of the scallion, lime juice, cayenne pepper, salt and cilantro and toss until mixed.
Heat 2 tablespoons olive oil in a large skillet over medium-high heat until hot. Add shrimp and cook until just opaque. Season with salt, and drizzle with 1/2 tablespoon lime juice.
Spread avocado mixture onto the tostadas. Top with shrimp, sour cream (optional), diced tomatoes and scallion greens.
Serve with lime wedges.