Dont forget that Serbia had wery good army in period just before battle of Kosovo.I think you shoud base on more western style units like french Noble Chavalry or English Knights.Yet the light cavalry shoud be more like russian troops Boyar Sons or Druzhina.Yet the infantry was mixed and you shoud make it something like Dismounted boyar sons, Varangian guard, Dismounted Knights and Sergeant Spearman.
Bechause the chavalry was stronger i sugest you create a chavalry unit to be the unique one and call it something like Serbian Knights or Order of the Dragon of saint Djordje.You shoud think about creating a unit called Knights of death bechause they were formed after Kosovo battle to avenge prince Lazar (they actualy survived the battle).I'm holding fingers for this mode