In a large mixing bowl, combine all ingredients except tomatoes; reserve 6 shrimp for garnish. Mix well, cover and refrigerate at least 2 hours. Cut ¾ inch off top of each tomato and reserve. Using a teaspoon, scoop out tomato seeds and pulp. Turn tomatoes upside down and allow to drain well on paper towels. Dice one cup of sliced tomato tops and mix in with chilled vegetables. When tomatoes are well drained, stuff each with an equal amount of shrimp and vegetable mixture. Serve on multicolored lettuces as a salad or appetizer; top each stuffed tomato with a shrimp.