In a large saucepan, combine the sugar, vinegar, oil, water, salt, pepper and, if desired, pepper flakes. Cook and stir over medium heat until mixture comes to a boil. Cook 2 minutes longer or until sugar is dissolved. Cool to room temperature, stirring several times., In a large salad bowl, combine the coleslaw mix, tomatoes, onion, peppers and pickle relish. Add the dressing and toss to coat. Cover and refrigerate overnight. Serve with a slotted spoon.