Cook shells with tablespoon salt to 1 minute short of package time. Reserve 1 cup pasta water; drain.
Meanwhile, brown sausage in large skillet 7 to 8 minutes, breaking small, to at least 160°F. Drain fat, leaving about 1 teaspoon.
Add butter and garlic; stir over low 30 seconds. Add peppers, cream, milk, and black pepper; gently simmer 2 minutes.
Add pasta and 1/4 cup reserved water; toss 1 to 2 minutes until tender. Remove from heat; gradually add Parmesan.
Loosen with pasta water if needed; serve with sausage and peppers distributed among shells.