Bring a medium pot of salted water to a boil and cook 1-3/4 cups uncooked elbow macaroni according to package directions until al dente; add 1 cup frozen peas during the last minute of cooking.
Drain pasta and peas in a colander, rinse under cold water to stop cooking, and let them cool and dry while you prepare the other ingredients.
In a medium bowl, whisk together 1/3 cup Caesar dressing, 3/4 cup mayonnaise, 3/4 cup sour cream, 1/2 teaspoon garlic powder, and salt and pepper to taste (start with 1/2 tsp salt and 1/4 tsp pepper).
Dice 1/2 cup bread-and-butter pickles, 1/2 cup red bell pepper (about 1/2 pepper), 1/2 cup celery (about 2 stalks), and 1/2 cup sweet or red onion; measure out 1 cup matchstick carrots.
Flake and thoroughly drain 2 (5-ounce) cans of tuna and add to a large mixing bowl with the cooled pasta and peas and all diced vegetables.
Pour the dressing over the salad and gently fold until everything is evenly coated; taste and adjust seasoning, adding up to another 1/4 teaspoon salt if needed.
Chill the salad in the refrigerator for 20–30 minutes before serving and stir gently again just before serving.