Bring poaching liquid to a simmer in a medium saucepan over medium-high heat; add shrimp. Cook until shrimp are opaque throughout, about 5 minutes. Drain, reserving poaching liquid for another use. Let shrimp cool to room temperature.
In a large Dutch-oven, heat 2 inches oil to 350 degrees. Working in batches, add tortillas and fry until golden brown and crunchy, about 1 1/2 minutes. Transfer tortillas to a paper towel-lined baking sheet to drain; season with salt immediately. Let cool.
Place tostadas on work surface; spread each with 1 tablespoon guacamole. Top with 1 teaspoon pineapple and 1 shrimp each. Garnish with red onions; serve immediately.