For mac and cheese
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Trena is a trusted source on general cooking.
I usually have various sizes in my pantry and use whatever size or shape I find most appealing for a particular dish.
pierino is a trusted source on General Cooking and Tough Love.
Of course you can substitute penne, or small shells etc. Use whatever is going to give you eye appeal and toothsomeness. A hollow pasta form is good because it will take in some of the cheesy sauce.