In a medium saucepan, cover the rice with 1 3/4 cups of water and bring to a boil. Cover and simmer over very low heat for 12 minutes. Remove from the heat without lifting the lid and let stand, covered, for 5 minutes. Using a fork, fluff the rice, then cover and set aside.
Meanwhile, in a medium saucepan, cover the shrimp shells with 4 cups of water and bring to a boil. Simmer over low heat for 10 minutes. Strain the broth and return it to the saucepan.
Heat the oil in a small skillet. Add the garlic; cook over low heat until golden, about 2 minutes. Add the crushed red pepper. Scrape the garlic oil into a bowl.
Bring the shrimp broth to a boil. Add the snow peas and tomato and simmer over moderately high heat for 1 minute. Add the shrimp and cook just until opaque throughout and curled, about 1 minute. Stir in the garlic oil, fish sauce, cilantro, lime juice, lime zest, pepper and scallion. Spoon the rice into deep bowls, ladle the soup over it and serve.