Salat behind Shiites

Assalamualaykum.Dear scholar, I am a sunni muslim. I wanted to know, am I allowed to pray behind a Shiite muslim? I asked a local scholar in a nearby mosque and he said that it IS allowed to pray behind them, because Shiite muslims in general aren't kuffars. However, if you're sure that the certain individual's believes indeed takes him out of the fold of Islam (he's a kafir), then I am not allowed to pray behind that individual shiite. What are your thoughts on this? And what if I know for a fact that he's a kafir Shiite muslim, and he wants to pray behind me? Should I stop him, or just allow him to do so? Thanks for your time, I appreciate it. JazakAllahkhair!

As-salamu `alaykum. Please inform me of the legality of praying in a Shiite mosque. I am an Egyptian Muslim, but I work these days in Iran.

As-salamu `alaykum, I attended an iftar in a Ja`fari Shiite association, and then I performed Maghrib and `Isha' Prayers behind a Shiite Imam. Is my Prayer acceptable? They made some differences in Adhan. They also put something (like a stone) in front of them while offering Prayer. Also, in the Prayer itself, they made a few differences.Please let me know whether my Prayer was valid or not. Jazakum Allahu khayran.