Preheat the oven to 325 degrees F. Heat the olive oil in a large skillet over medium-high heat. Add the carrot, scallion whites, chile powder and half of the tomatoes. Cook until the tomatoes release their juices, about 3 minutes. Add the sausage and cook, breaking up the meat, until it begins to brown, about 5 minutes. Add half of the scallion greens and cook until most of the liquid has evaporated, about 5 minutes.
Meanwhile, put the taco shells on a baking sheet and warm in the oven, about 6 minutes. Mix the mango with the lime juice in a medium bowl; sprinkle with chile powder. Add the remaining tomatoes and the scallion greens and toss.
Fill the taco shells with the sausage mixture and top with the lettuce, cheese and mango salsa.