Add water to a large pot with about 1 tablespoon of salt. Heat until boiling, then add the raw shrimp. Boil for 3 minutes or until pink and firm. Remove immediately from hot water and place into bowl of ice water to cool down quickly. Once cooled, drain the shrimp and pat down with paper towels. Remove tails and dice into bite-sized pieces. Place into large bowl.
Juice limes and orange and place into bowl with shrimp. Add in diced red onion and jalapeno, along with 1/2 teaspoon of salt and 3/4 teaspoon garlic powder. Stir together. Cover and place in refrigerator to marinate for 1 hour.
While the shrimp marinates, make the tostadas. Add the canola oil to a heavy-bottomed skillet (such as a cast-iron skillet). Heat over medium-high heat until hot. Add one corn tortilla at a time and fry until browned on one side. Flip and then fry until other side has browned. Remove from oil and drain on paper towels. Sprinkle with a little salt. Repeat with the rest of the tortillas.
Remove marinated shrimp from fridge and add diced cucumbers, diced tomatoes, and cilantro. Stir together. Give it a taste and add more salt, if necessary.
To assemble the tostadas, first make the spicy mayo. Mix the mayo and hot sauce together in a small bowl. Spread about 1 teaspoon onto each tostada. Using a slotted spoon, scoop about 1/4 cup of ceviche onto the tostada, being careful not to get vinaigrette on the tostada. Slice the avocado and top each tostada with a slice. Serve tostadas immediately.