Is it permissible to pray behind imams of a Sufi mosque who openly supplicate after every prayer and intercede through the Prophet, peace and blessings be upon him, in their private gatherings?

The fact that the mosque's congregants raise their voices in supplication after every prayer does not mean they are Sufis. They may be ignorant of the Sunnah. However, the private gatherings and their supplication through the Prophet could strengthen the assumption that they are Sufis.

Supplication through the Prophet is of two types: the first is major shirk (polytheism), which is supplicating to his essence and invoking him; the second is an innovation (bid'ah) and not shirk, which is supplicating through his status and right.

These individuals should be taught gently. If they respond, then praise be to Allah. If they do not, then prayer behind them is valid, while being keen to pray behind a Sunni imam.

It is stated in "Fatawa al-Lajnah ad-Daimah" that prayer behind innovators is not valid if their innovation involves shirk. However, if their innovation does not involve shirk, then prayer behind them is valid, while being keen to have an imam who is not an innovator.

Strive to establish a mosque founded on the fear of Allah and the Sunnah of His Prophet.