In reference to the actual issue, I am not a person of knowledge so do not want to appear as giving out rulings but the Qur'an says:
"And do not marry Al-Mushrikaat (idolatresses) till they believe (worship Allah Alone)"
So if a person was considering marrying a Shi'ah, and their belief went beyond the above ayah, then I am sure you can determine the rest. I don't fully know the different types of Shi'ah, but similarly to the actual People of the Book, I wonder if there are any Shi'ah's left who only preferred Ali (radiyAllahu anh) as the third caliph instead of Uthman (radiyAllahu anh) instead of what the Shi'ah (Rafidi in this instance from my understanding) belief has developed into. I am guessing here but I assume this where the ruling which forbids such a marriage stems from because of incompatibility in core belief.
The other point regarding the basis/authority of scholarly rulings, then, after the Qur'an and Sunnah, we refer to the people of knowledge, being the scholars but I don't think (from my limited knowledge) this is an issue where you can say there is scholarly dispute or that it is the opinion of one scholar only, because the Qur'an clearly states, do not marry idolatresses, and like I alluded to previously, I guess, this is where the ruling stemmed from.
But the Shia I communicated with deny any god other than Allah. Just as there are Christians who deny the claim that Jesus is anything other than a prophet (example: Jehovah's Witness)
I don't think many people here studied world religions in college...thus it explains the limited knowledge on such matter. I thought Allah commanded us to learn???