Pellegrino is right that this thread is not the place to discuss the comparative virtues of Christianity and Islam. All Ill say is that the shape and nature of religions change drastically depending upon where and when they are practiced. Islam under the Umayyad Arabs was understood quite differently than it would be in al-Andalus a hundred years later by Arabs, Berbers, and Iberians. Similarly, the Christianity of the Papacy in 1215 differs quite a bit from the Christianity of Syrian Christians in the eighth century.
As to the place of Islam in this (potential) clan, personal belief in Islam would not be a requirement for entrance. However, you cannot accurately portray an Arab or a host of other racial groups in the Middle East, Africa, and Europe without integrating Islam as part of their identity. As such, Islam would play a large part in the naming structure and RP of the group and would define what armor is acceptable for members. It would not, however, be the Islamic parallel to the crusading clans. Primarily because, although Islam understood the idea of jihād to be a religious duty, there was never the proliferation of dedicated orders that characterized crusading. Additionally, while a crusade was seen as an individual, penitential duty to ones soul, jihād (the lesser jihād, anyway) was usually seen as an obligation of the Muslim polity.
So, for those tl;dr folks: Yes, Islam would be an important part of the RP of this clan. Yes, it would be awesome to do some RP organization with the crusader clans.
No, this clan would not be about jihād and nothing else.
So, for those interested, please let me know in the replies or PM me personally.