Instead of the feudal system, the Turks had the timar system. See my essay on it here:

https://forums.totalwar.org/vb/showthread.php?t=69648

In the timar system there were no slaves, and instead there were sipahis. So there shouldn't really be a slave unit. On the other hand, the Ottomans used units of men, generally crazy people, freaks, or emigrating Turks (ottomans did not equal Turks.) in front lines to break the enemy formations before the main, trained forces attacked.

Turks were originally emigrating people, and after the establishment of the Ottoman government, the ones that continued the emigrating culture possessed a lot of problems for the Ottomans. Therefore, the government would usually just move them around for their benefit, or just use the ones that made trouble in battles. I am not 100% sure on which periods this was present in the Empire, but it did exist at some point.