I think, the possibility of a full scale conflict is close to nil. Other than the Pakistan nuke factor, there are some other factors that make such a scenario quite impossible.
1] Presence of a large international force in that area. I'm sure one of the US, UK, France will be able to convince Pak, Iran.
2] China - It is the closest that both these nations have to a friend. China has friendly/trade/defence relations with both Iran and Pakistan. A conflict, small or big between Iran and Pak would, I think, be detrimental to China's interest in this region. And China wields considerable influence in both these countries.