Are Sufis Muslim?
For marriage, common sense suggests that one should consider compatibility in beliefs. Sharia does not give you or I the right to decide on someone else’s religion. It is each person’s own individual right and the right to accept or reject that person as a Muslim belongs to God. There is nothing for you or I to decide here.
The Sharia prohibition in marriage is in reference to shirk (polytheism). The rest is up to one’s own sensitivity and how much importance they give to their own belief system.