As for Shia and Sunni coming together, it's possible though a bit unlikely... They've not been on good terms for quite some time, even Catholicism, Orthodoxy and Protestantism failed to come back together after hostilities died down.
Even if they have the Sunni/Shia division, the Muslims will unite when they face a common enemy. Have you ever heard of any Sunni Shia clashes in countries where Muslims are minority? These clashes only happen in nations where they are more than 90% of the total population. The same goes with Catholicism/Orthodoxy/Protestantism.