Faction names are in the ".../Rome - Total War/EB/Data/text/expanded_bi.txt", but you have to change a couple dozen references.

Starting cities are in the ".../Rome - Total War/EB/Data/world/maps/campaign/imperial_campaign/descr_strat.txt". Be careful though, it is very easy to mess up when changing starting cities.

To allow units to another faction you have to change multiple files, namely the ".../Rome - Total War/EB/Data/descr_model_battle.txt" to make sure they have a skin assigned to the model, the ".../Rome - Total War/EB/Data/export_descr_unit.txt" to make sure that the new faction is included on the recruitment line for that faction, and the ".../Rome - Total War/EB/Data/export_descr_buildings.txt" to assign recruitment for the unit (easiest just to copy the recruitment lines for the old faction and edit them appropriately). You may also have to copy the UI files to a different folder, for the full effect.