Have you also asked this question over on the steam workshop forums? If not someone else is doing exactly the same thing as you
Anyway, if you want to change the units of a faction you need to change the units_to_groupings_military_permissions and the building_units_allowed.
You will need to delete all the old enteries for the rebels and make your own adding what units you want to the faction first in the units_to_groupings_military_permissions and then add them to what buildings you want to recruit them in the building_units_allowed, note that you will need to add units to all higher levels of buildings unless you want to replace them at higher tiers (i.e Germanic nobles recruited from a level 2 city would also need to be added to level 3 and level 4 if you want to continue to recruit them as the campaign goes on)
Unless someone knows an easier way?