The Eleutheroi are all the factions that didn't make it, plus anyone who rebels against you. This means they have access to almost all units.
The Eleutheroi Unit pages list the most common troop types within a geographical area and any unique units available there. Remember that the unit pages do not list every available unit the Eleutheroi has.
Note!! Some units can be recruited by many factions over a wide area, almost all of them either Celtic or Hellenic. That's why there is a Celtic & Hellenic category.