I wouldn't say that the Khoor would represent purely the Mongols, at least in my mind. Rather, I look at them as a representation of the Pecheneg, Avar, Huns, Kipchak, Baglvalal, and, of course, the Mongols/Tartars in addition to many others.
Regardless, the name Khoor seems derived from the Khurrites, which is another name for the Hurrians, which are one of the ancient antecedents to modern day Armenians. While the Khoor in game look purely Asiatic, the name leads me to think of them as a singular representation of the horsemen of the 'oriental' tribes, as they were thought of then. Most of those tribes were, in actuality, from Anatolia (modern day Turkey) and the areas surrounding the Black and Caspian sea, and up into the Don river basin.
From a Russo-centric point of view, they seem completely logical to have in the game.
That being said, given the power and influence of the Umayyad Caliphate in Europe (specifically in Spain), an Arabic analogue would certainly make sense.
Might be a bit late to add more races, though.
Life is Iqta.