I am not aware that nocks come in different string groove sizes, except by accident. While you can heat standard nocks and widen them to fit, they won't fit as consistently as stock nocks. Better to change the serving to fit the nocks you want to use. If this is a custom string made to your specs, you should be able to send it back with a sample nock, and they will change the serving for you. Otherwise, it's not too difficult to learn to put on a new serving yourself, although it does require a little investment in time and equipment. It will probably be your first step into building your own string, which is a useful thing to know how to do.