Place onion and carrot in a 3-1/2- or 4-quart slow cooker. Place beef on top of vegetables. Sprinkle with 1/4 cup cilantro, the crushed red pepper, and salt; add the undrained tomatoes and water.
Cover and cook on low-heat setting for 7 to 9 hours or on high-heat setting for 4 to 5 hours. Remove meat from cooker. Using 2 forks, shred meat. If desired, cut meat to make shorter shreds. Drain vegetable mixture well; discard liquid. Transfer vegetable mixture to a food processor bowl or blender container. Process or blend with several on/off turns until chopped.
Return meat and chopped vegetable mixture to slow cooker. If using low-heat setting, turn to high-heat setting. Cover and cook for 15 to 30 minutes or until heated through. Spoon mixture into miniature taco shells. Top with mango and desired toppings. Makes 24 tacos.