Pakistan need to make sure that Shia's do not torpedo national interest of Pakistan which is an overwhelmingly Sunni country (around 85%), just to do a favor for Iran, the country that is closest to the heart of all Shia of the world, as it is their main base of power in the world.
Shah was a moderate, but unfortunately he was a Western puppet and then the Mullah's are extremist per excellence, not being Western puppets does not help their cause one bit. One of the biggest advantage the Iranians going for themselves is that they are nationalist and Imperialist first and foremost, even before being Muslims or Shia. So they use whatever ideology they follow to advance their Persian imperialist agenda. Using their Shia faith is good for this agenda, so they use it and even the Zoroastrians or Seculars or atheist Iranians will support these mad Mullah's as long as they see Persian imperial agenda being advanced.
Arabs on the other hand are relative simpletons compared to Iranians, and they are not united in one country, after the breakup of Ottoman. So even if they have the advantage with numbers and resources, they seem not to do well. One very pertinent example I will give is the spat between Gulf Monarchs and Muslim Brotherhood, and sneaky Iranians sure enough find a way to take advantage of this Arab disunity with their support for MB and Hamas.
And the West, Chinese, Russians and Hindu's all, powers of any consequence would love nothing more than to see their enemies fighting and exhausting their resources, so Shia-Sunni fight is of greatest benefit to all of them. Iran-Iraq war, post-Saddam ongoing sectarian war in Iraq and ongoing Syria sectarian conflict are good examples.
What Sunni's of the world need to understand is that Iran will not cease and desist, till it is thoroughly humiliated and defeated in its imperialistic agenda. Only after their total failure, the Iranian people may rise up and remove the Mullah's from power. In this fight, Sunni's should ally with whoever is willing to help the Iranian people remove this curse from their country - the West, Israel etc. Sometimes it is better to ally with enemies to remove a more immediate and manageable threat.
China and Russia should be warned to stay out of this fight to help the Iranian people remove the curse that has befallen on them.