Bring a large pot of salted water to a boil and cook the pasta shells or rotini according to package directions until al dente. Drain in a colander and rinse briefly under cold water to stop cooking, then set aside to drain fully.
If using fresh salmon, preheat the oven to 400°F (200°C), pat the salmon dry, season with salt and pepper, and bake for 12-15 minutes until it flakes easily. Let cool slightly, then flake into bite-sized pieces with two forks. If using canned salmon, drain well and flake with a fork, removing any bones or skin.
In a medium mixing bowl, whisk together the mayonnaise, sour cream, lemon juice, Dijon mustard, fresh dill, garlic powder, salt, and pepper until smooth and fully combined. Taste and adjust seasoning as needed.
Dice the celery, cucumber, red bell pepper, and red onion into small, even pieces. Measure out the thawed peas and set all the vegetables aside together.
In a large mixing bowl, combine the cooled, drained pasta with the celery, cucumber, red bell pepper, red onion, and peas. Pour the dressing over the pasta and vegetables, then toss gently with a rubber spatula until everything is evenly coated.
Add the flaked salmon to the bowl and fold gently with a spatula, turning the salad over on itself rather than stirring hard. Stop folding as soon as the salmon is distributed to avoid breaking it into tiny pieces.
Cover the bowl tightly with plastic wrap or transfer the salad to an airtight container. Refrigerate for at least 1 hour to let the flavors come together and the dressing thicken. Give the salad one more gentle toss right before serving.