Well. Let me think.
There has to be some units "shared" by Islamic culture. Ghulams for instance. Every sultan had to have an army of well trained slave soldiers to gain power enought to split his people form Caliph.
Im pritty sure there should be other units used by "all" of Islamic word, but recruitable in only certain areas.
(i dont know of Immamate, but if they grew in power in the same way as Sulatanates [military dominance over Caliph and loyal Ghulams who had high social rank] they should be allowed to train ghulams if were to get access to required terrain).
I know it "can" look funny in custom battle as every Islam faction will have ghulams :p