To prepare fresh peppers (whether hatch chiles or poblano pepper) for this recipe, roast them over an open flame on the stovetop until well charred; place them in a plastic bag for 10 minutes, then peel the skins and remove the stems and the seeds.
Place prepared chiles in a blender along with 3 tablespoons water, lime juice, vegetable oil, garlic, thyme, cayenne pepper, cumin and salt. Blend until smooth. Toss in a large bowl with shrimp until coated. Let sit 15 to 30 minutes.
Meanwhile, heat about 1/4 inch of vegetable oil in a large skillet over medium-high heat. Fry 1 tortilla at a time, flipping once, 1 minute until crisp and golden brown. Transfer tortillas to a paper towel-lined baking sheet to drain. Remove skillet from heat; drain oil, then return skillet to stovetop.
Make the guacamole: Halve and pit the avocados; mash until mostly smooth. Add white parts of green onions, cilantro, lime juice, salt and cayenne pepper. Stir to combine.
Heat reserved skillet over medium-high heat. Cook shrimp 2 minutes until warmed through.
Assemble the tostadas: Spread guacamole on top of each tostada, then top with cooked shrimp. Top with a dollop of sour cream, chopped tomato and reserved green parts of green onions. Serve with lime wedges.