That recipe is a little bit weird. In a traditional semolina dough nothing but water and semolina is used. You don't salt it because you're going to salt the water that it cooks in. And you definitely don't add any fillers like parmesan. That goes on the pasta, not in it. For two people you would be looking at 400 grams of semolina and 200 grams of warm water. Adjust one or the other as is needed after you start working the dough.

Anyway semolina is the traditional flour for pasta. In America a lot of the cheaper pastas use durum but if you are going to make it at home then semolina is definitely best. Durum or AP flour can result in a slimy coating that won't let sauce stick. And cavatelli is all about creating something that will hold even the lightest sauce.