Cook the pasta in a large pot of salted boiling water according to package instructions. Drain and rinse under cold water. Set aside to cool completely.
In a large bowl, combine cooked pasta, chicken, shrimp, pepperoni, onion, red bell pepper, celery, peas, and pecans.
In a separate small bowl, prepare the dressing by whisking together the mayonnaise, Ranch dressing, Dijon mustard, vinegar, sugar, tarragon, dill, and garlic powder. Taste and season with salt and pepper.
Pour the dressing over the salad; toss gently to coat. Cover and refrigerate for at least 1-2 hours, or overnight.
Just before serving, give it a stir, add extra mayonnaise if the salad seems dry, and season with additional salt and pepper, if necessary.