Toast the chiles in a dry skillet over medium heat, turning frequently, until they start to get speckled.
Transfer the chilis to a bowl and add enough hot water to cover them. If they float, set a small plate or saucepan on top to keep them submerged. Let them sit for about 15 minutes.
In a blender or food processor, add the chiles, 3 tablespoons of the soaking water, 1½ tablespoons of lime juice, olive oil, garlic, oregano, cumin, ¼ teaspoon of cayenne pepper, and ¼ teaspoon of salt. Purée until smooth.
Discard the remaining soaking water, then place the shrimp in the same bowl. Pour the chile mixture over the shrimp and toss to coat.
Cover the bowl and refrigerate for 20 to 30 minutes.
In a skillet big enough to hold a tortilla, add ¼ inch of vegetable oil and heat over medium-high heat.
When the oil is hot, add one tortilla. Cook until golden brown and crisp on both sides, about 1 minute, then transfer to paper towels and sprinkle with salt. Repeat with remaining tortillas.
Halve and pit the avocados, then scoop the flesh into a small bowl. Add the white parts of the scallions, 1½ tablespoons of lime juice, the remaining ¼ teaspoon of cayenne pepper, and ½ a teaspoon of salt.
Heat 2 tablespoons of vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the shrimp and cook, stirring occasionally, until just opaque, about 5 minutes.
Remove the shrimp from heat and drizzle with the remaining ½ tablespoon of lime juice.
Spread the avocado mixture onto the fried tortillas and top with shrimp. Add your desired toppings and serve with lime wedges.