Agree with Orion.
I would opt for changing the serving to fit the most commonly used nock size, and then if the nocks that are fitting too tight are not that size, changing those nocks as well. I find that more than 90% of the nocks I come across fit the same size string. Unfortunately, I don't know how that size is measured. But an example of nocks that fit the same string pretty closely would be an Easton G nock, a Bohning F nock, or a Gold Tip GT nock.
Choose the serving size such that the arrow will snap on easily, not fall off of the string under its own weight, but will fall off if you give the strin a sharp tap.