By the time of the crusades, the vast majority of professional soldiers were slave-recruited. Since it wasn't permissible to enslave Muslims, these slaves came from the fringes of the Muslim world or beyond, which meant that professional soldiers weren't Arabs. Settled Arabs served in town militias, and Bedouins sometimes acted as scouts and raiders. The latter weren't dependable, since their loyalty was primarily to their tribes, and were willing to fight on whatever side they saw the most profit in. Arabs who were career soldiers seem to have been a minority. The Fatamid heavy infantry I previously mentioned were sub-Saharan Africans. Their professional cavalry were Turks and Rum (Anatolian Greeks, Armenians, etc.).
William of Tyre described a Muslim army that defeated Amalric of Jerusalem in Egypt as consisting of "twelve thousand Turks of whom nine thousand were heavily armored and wore helmets, while the other three thousand used only bows and arrows". Note the ratio of heavy to light troops there.