Bring a large pot of lightly salted water to a boil; cook the rotini at a boil until tender yet firm to the bite, about 8 minutes. Drain and reserve the water. Pour water back into the pot. Add shrimp to the boiling water and cook while stirring until pink, 2 to 3 minutes. Drain shrimp.
Combine lemon juice, vinegar, shallot, and garlic in a cup. Beat in olive oil until well combined. Season dressing with salt and pepper.
Combine cooked rotini, shrimp, mixed greens, and avocado in a bowl. Toss salad with the dressing.
