First off, just so we're on the same page, only northern Italy (Cisalpine Gaul) is included on the game map. The campaigns we're doing now are focused on the frontier of the "empire" and so the territory controlled by Rome at the beginning of the game is a relatively small percentage of the map.
With that said, the Roman factions work quite differently depending on whether you're playing the campaign or the sandbox. In the campaign, you play as your own Caesar faction that only has control over the units and resources that directly affect the events at hand. Where important to the campaign, other Roman resources are controlled by allied AIs. This includes cases where Caesar handed over control to subordinates who led campaigns in other parts of the map and then rejoined Caesar's forces later.
In the sandbox, we currently have the Roman territory split into two factions: Cisalpine Gaul and Transalpine Gaul. We did this mostly to keep them comparable in size to individual Gallic, German or Briton factions, but that may change as we get further into playtesting. We are also planning multiple sandbox configs, so we might also include a unified Rome against a unified Gaul for example.
As far as Roman politics are concerned, the gameplay in Hegemony remains focused on military campaigning, but the events going on back in Rome did play a key role in Caesar's decisions and we've tried to draw those connections through the cutscenes and objective narrative.