Try out small, don't know to much about modding. I think it would be better to have an Abolitionist Slave Revolt faction and not a slave faction. The more I think of this the less it makes sense in normal M&B Warband game play, where there are different factions that fight over territory. I say this because of a historical view of slave revolts, where slaves would revolt for freedom and go somewhere they would be safe. Like the slaves fleeing to the North, Spartacus trying to flee Roman Empire.
I think you should either choose a setting from "Bleeding Kansas" where Pro and Anti Slavery forces battled or like in Haiti. Haiti has a background where you could have French, British, Spanish, Rebellion faction and even a African Slave faction. In the "Bleeding Kansas" theme it was mainly the two factions, but you could make some thing up like Pro Slave, Anti Slave which was for the USA as a state. But maybe have a separate faction for a new country, maybe a Native American faction.