Heat oven to 450 degrees. Cover a large baking sheet with foil. Lay tortillas on sheet and spray with cooking spray. Turn over and spray other side. When oven is heated, bake for 10-15 minutes or until light golden brown. They may still appear floppy, but will become crispy as they cool. Remove and place on plate covered with paper towels. Blot with a paper towel to absorb any excess oil.
In a small bowl, combine the cumin, garlic powder, pepper, salt, chili powder, brown sugar, and oregano. Put the olive oil in a ziptop bag and mix in all but 1 tablespoon of the spice mix. Add the garlic and shrimp; massage to coat. Set aside.
Heat a 12-inch sauté pan over medium-high heat, and, when hot, coat with cooking spray. Add the sliced onion and sauté until softened and fragrant and starting to brown, about 4 minutes. Add the chile and bell pepper and continue to sauté for 2 minutes. Add the remaining spice mixture and sauté until the onion and chiles are tender but not mushy, another 1-2 minutes. Transfer to a bowl. Return the skillet to medium high heat and add the shrimp. Cook, stirring frequently, until the shrimp are cooked through, 2-3 minutes. Add the sautéed vegetables; toss to combine and reheat.
Spread a small amount of guacamole on tostada. Spread a small amount of sour cream over guacamole, then add some lettuce. Add some sauteed vegetables and top with shrimp. Sprinkle with chopped cilantro.