It is not just the Hazaras that coalition forces need to worry about.

Jamestown Foundation - Iranian Involvement in Afghanistan

More than a decade ago, while mujahideen leaders were toppling the Moscow-backed Afghan leader Mohammad Najibullah, it was predicted that a strong Sunni fundamentalist regime in Kabul could come into conflict with Shiite Iran. This fear led Tehran to support groups such as the Shiite Hazara parties and the influential Tajik commander Ismail Khan in Herat province. When the Taliban finally gained control of Afghanistan, Iran referred to the development as a Sunni and U.S. plot to isolate Iran. The relationship between Kabul and Tehran took a more serious hit when Taliban forces killed seven Iranian diplomats who were serving in Mazar-e-Sharif in August 1998. This Taliban action led Tehran to announce its open support for all forces that would resist the Taliban and to increase its activities to bring anti-Taliban factions together. The most notable act by Tehran was to allow the influential Pashtun leader, Gulbuddin Hekmatyar, to be stationed in Iran.
While Hekmatyar and Iran had a falling out in 2002, I do not think it is completely unlikely for them to create another friendly relationship with the US as the common enemy. Additionally, since Iran already has a history of supporting non-Shiites like Hekmatyar, they are just as likely to establish relationships with other Afghani and Pakistani warlords, regardless of ethnic or religious affliation, if they have not already.