Q. Can we pray behind an imam who goes and follows dargah and when is it compulsory to do dua, after farz salah or after sunna salaah?
A. Those who visit the dargah and follow the rituals that take place at these places are engaged in grave sins that are tantamount to shirk. The Beliefs and practices of these people are totally against the teachings of Islam and are in opposition to the established creed of Ahlus Sunnah Wal Jamaah.
As such, when you know that the Imam follows the customs and practices of the dargah then you should not perform salaah behind him.
With respect to dua, this is a very great act of worship as mentioned in the Ahadith. However, it has not been made compulsory (farz) at any given time. A person has the option of making dua when he wishes to do so. It can be after the farz salaah or after the sunnah salaah.
And Allah knows best.
Mufti Waseem Khan