What is the ruling on an Imam who selectively chooses what he likes from the doctrines of the Shiites, Sufis, and Sunnis?

This man is gravely mistaken, for matters of creed are not subject to people's whims. Rather, it is incumbent upon every Muslim to believe in the creed that conforms to what the Prophet, peace and blessings be upon him, and his Companions were upon. This is the saved sect about which the Prophet, peace and blessings be upon him, spoke. Sheikh al-Islam Ibn Taymiyyah clarified that this sect is the People of the Sunnah and the Community (Ahl al-Sunnah wa al-Jama'ah), and they are the victorious group. He emphasized that adhering to the Sunnah is salvation, and that following the Sunnah is the straight path that leads servants to Allah. He mentioned that all sects that differed from what the Companions and the People of Hadith were upon are among the paths about which the Prophet, peace and blessings be upon him, warned. Therefore, it is incumbent upon every Muslim to correct his creed to what the Prophet and his Companions were upon, and to avoid all creeds that were innovated after them. This man is in grave danger if he does not repent, rectify his creed, and adhere to the pure Sunnah.