Is there any way to assimilate the factions you conquer to lower the garrison required to keep peace?

no, tear down the walls and take mercs for secure

some missions will allow the city to became a city of macedonien (if u play campain)

I asked about modding a while back and Rick gave me a solution to change some of the files so that hostility can be modded. Set that at zero and you require much smaller garrisons. Try searching for that old thread.

i think this is not good for realism.
They are hostile citys and its just fair to send garrisons.

in my opinion the gameplay would become to easy.

in the next houres i will face this problem because i engange to conquer theben and athen
and that means a lot of garrisons but i will handle it

i think it is just a madder of gold you have

i have 5 peace treaty wich gives me additional founds of 500 gold.
for the moment i have no prblems with troops to hold hostile citys

maybe i change my mind but for now i think is not necessary