Any noodle with a long or large shape will make pasta salad much harder to eat while standing up and chatting. A major convenience factor is smaller noodles.
While a good store-bought dressing won’t ruin your pasta salad, homemade will be a better choice because it gives you control over the ingredients and flavor.
Since pasta salad spends some time in the fridge, butter is a bad addition. The butter will congeal, and you’ll end up with a blob of pasta that can hardly be pried apart.