Low-fat milk or mayonnaise will make the dressing too thin.
Whisk milk, mayonnaise, sugar, pepper, and ½ teaspoon salt together in bowl. Reserve and refrigerate 1 cup dressing for finishing salad. Set both aside.
Bring 4 quarts water to boil in large pot. Add 1 tablespoon salt and pasta and cook until very soft, about 15 minutes. Drain pasta in colander and shake to remove excess water. Transfer pasta to large bowl, add vinegar, and toss until vinegar is absorbed. Let pasta cool for 10 minutes.
Stir remaining dressing (larger portion) into pasta mixture until combined. Stir in scallions, carrot, and celery until combined. Cover and refrigerate until fully chilled, about 1 hour. Stir in reserved 1 cup dressing. Season with salt and pepper to taste. Serve.
