I’m not saying they’re essential but they do have a strong Persian connection and Ghulams would work just as well as a minor civ unit so it’s an option to consider.
Personally I’d lean towards keeping Qizilbash as a native unit and just giving Persia good shipments of them.
Ghulam as a standard unit could also be shared with an Afghan civ so that’s a plus. But I’m not familiar with their cavalry so maybe they have better options.