I am replacing seats and stems in tub/shower. The old one is Sterling--the new faucet set it a PricePfister. The PF diverter stem doesn't fit into the seat (the hot and cold stems do). Can I go to another size diverter with PF?

If I understand you correctly you are trying to put a PF stem into a Sterling valve body. No can do. You have to replace the Sterling with its own parts. Your best bet is to take your original parts to a plumbing store and let them give you the right replacement stems and seats.