Well, we decided to make Islam a little more flexible at start. Historically it starts out as a heresy of Arabic Paganism but if another religion is dominant in Mecca then Islam becomes a heresy of that religion instead. If you want further info on this, please find it in the earlier discussion regarding Islam. But you can rest easy, I think we never mentioned anything about changing the titles/names
As a fact, the Muslim army that was sent to conquer Mesopotamia was only 18 000 strong, while, even after a long war, the Persians actually fielded an army equal in size. Twice as large if you count its Arab Allies.