Make the shrimp salad:
Fill a large bowl with 10 cups cool water; stir in 1/4 cup salt (it should taste like seawater). Add shrimp and soak 30 minutes, then drain.
Fill a medium pot with 8 cups water; stir in 1/4 cup salt. Bring to a boil, add shrimp, and immediately turn off heat. Let sit until pink, opaque, and barely cooked through, about 3 minutes. Drain, transfer to a large bowl, and let cool.
Add tomato, onion, cucumber, chiles, lime juice, and cilantro to shrimp. Season with salt. Chill until ready to use.
Assemble the tostadas:
If frying tortillas, fit a large skillet with candy or deep-fry thermometer; pour in oil to measure 1". Heat over medium-high until thermometer registers 375°F. Fry tortillas in batches until crispy, turning every 30 seconds, about 2 minutes total. Drain on paper towel-lined baking sheets and season with salt.
Spread 1 heaping Tbsp. Hold-the-Lime Guacamole on a tostada shell. Top with 1/4 cup shrimp salad, then lettuce, crema, queso fresco, and salsa. Repeat with remaining tostada shells. Serve at room temperature.
Shrimp salad can be chilled for up to 1 day. Bring to room temperature before serving.