In a bowl, mix bread crumbs, sirloin, scallions, chopped mint, egg, oil, half of garlic, 1 tsp. cumin, cayenne, 2 tsp. salt and 1/2 tsp. pepper. Use your fingers to combine well. Form into 40 1 1/4- to 1 1/2-inch meatballs. (You may cover and refrigerate meatballs overnight at this point if you'd like.)
In a slow cooker, stir tomatoes, mint sprigs, remaining garlic, remaining cumin, 1/2 tsp. salt and 1/4 tsp. pepper. Gently stir in meatballs. Cover and cook on low until meatballs are cooked through, 3 to 4 hours. Remove and discard mint sprigs. Skim oil from top of sauce.
Divide meatballs and sauce among pitas, if desired; stuff with cucumbers.