Did Sai Baba appear to his devotees in disguise? I read something about a fakir and another saint.

Yes, the texts suggest that Sai Baba would interact with his devotees in various forms. Chapter 33 provides two excellent examples of this. The first involves Karnik's encounter with the saint Narsing Maharaj, where the saint's sudden demand for a rupee felt so powerful that Karnik perceived it as Sai Baba himself accepting the coin. The second, more direct example is the story of Appasaheb. After Appasaheb silently promised ten rupees, a stranger-fakir appeared to him. The narrative strongly implies this was Baba in disguise, coming as a "pretext for Dakshina." This shows that Baba could use other saints or disguises to test, teach, and interact with his followers.