Why not? I mix hatchery chicks which are NPIP with chicks I hatch from my flock which is not NPIP. None of them are vaccinated for anything. They do fine.
I don’t sell my eggs or chickens across state lines and I don’t take my chickens to any shows or swaps. If I ever decide to start selling hatching eggs I’ll probably go through the course and become a certified NPIP tester so I could certify my own flock. But until then, if ever, I’m not worried about it.
The way I read the Original Poster's question, the issue was what diseases might the non-NPIP chicks get from the vaccinated NPIP chicks that are now carriers of the diseases NPIP required them to be vaccinated for. How dangerous are the NPIP chicks to the non-NPIP chicks. Since the NPIP chicks are not required to be vaccinated, the question is now moot. There are no issues because of NPIP.
I think it was a reasonable question and I hope Luckydux has gathered that NPIP does not require vaccinations. Luckydux, if you are getting NPIP chicks, contact the source and talk to them about vaccinations. They are the only ones that know if they vaccinate for anything or not. If you are getting non-NPIP chicks, talk to that source too. Don’t take anything for granted. It doesn't matter if 20,000 don't and 1 does. You might be talking to that one.