My solution is simple just script :

monitor_event CharacterTurnStart FactionType all
and CharacterIsLocal

and I_NumberOfSettlements seleucid = 2
add_money seleucid, 100000

and I_NumberOfSettlements armenia = 2
add_money armenia, 100000

and I_NumberOfSettlements parthia= 2
add_money parthia, 100000

....


and I_NumberOfSettlements seleucid = 1
add_money seleucid, 200000

and I_NumberOfSettlements armenia = 1
add_money armenia, 200000

and I_NumberOfSettlements parthia= 1
add_money parthia, 200000


.... goes on for each faction.
My scripting info is very limited so ask any experienced script modder.
you may create units but it will be torture to write its code because you have to specify the city or you have to give units randomly. so basically to add money for them to hire mercenaries is my humble solution other than giving each faction hording ability.