These are the Factions:

seleucid - Romani
greek_cities - Koinon Hellenon
thrace - Epeiros
macedon - Makedonia
numidia - Ptolemaioi
romans_julii - Arche Seleukeia
romans_brutii - Baktria
romans_scipii - Hayasdan
carthage - Pontos
pontus - Saka Rauka
armenia - Sauromatae
parthia - Pahlava
dacia - Getai
germans - Sweboz
scythia - Arverni
gauls - Aedui
britons - Casse
spain - Lusotannan
egypt - Qarthadastim
saba - Saba



You can also use some of the cheats in the link I posted, to kill enemy FM, f.e. by sending them by cheat on a far away territory, fight against them with whatever force of you using the auto_win attacker cheat and let them die, which they will when they cannot flee to friendly territory after a defeat. Or you could give cities of these factions to weaker factions, or you just could give the weaker factions many, many mnai and let them do what they do.