If slaves are meant to represent the peasants /serfs conscripted in the medieval era to fight using like pitchforks 'alongside' professional troops, then their role and function is to be cannon fodder. That being the case - I think the game should severely restrict the weapons and armor they may equip. Armor restricted to a max of basic leather, weapons to the tools used in their labor (like hoes, pitchforks, etc.).
If 'slaves' are meant to represent an actual class of sentient folks trained in warcraft, but ingrained to live with very limited freedom, then their use of bows and heavier armor makes sense. Consider the practices of the City State of Sparta in the Classical Greek era. Male children were taken from their birthparents and raised in military barracks, trained to be soldiers. They were slaves in the sense that they had few choices, choices severely circumscribed by their narrowly focused training / upbringing. So, empires that enshrine slavery as a foundational aspect of their societies certainly would have 'malleable' soldiers who would fight better than farmers / serfs conscripted right off the farms. Its a game, and the current options allow a player to role-play either of these choices, and the many shades in between. I mostly play Kingdoms. May I ask those of you who play empires, what combination of traits, equipment, etc., would you suggest to create troops to represent these two pools of 'troops?'