Praying in congregation behind a Shia imam

Answer

If the imam is considered a disbeliever, no. If regarded as a Muslim, then the salah is valid but disliked (makruh). There are many sects within Shiʿism, so anyone seeking a ruling should present the specific beliefs of that group from their own reliable sources. In short: if a Shiʿa individual believes in the fundamental tenets of Islam, does not believe the Quran has been distorted, does not deny the honor of Aisha bint Abu Bakr, does not reject or slander Abu Bakr and Umar or accuse them of false allegations, does not insult the Companions, and holds no beliefs that constitute disbelief, then such a person is regarded as a Muslim. However, if a person holds beliefs that amount to disbelief, then he would be considered outside the fold of Islam. *And Allah knows best.*