A couple things that may help here are adding more static (0 move) garrison units, and adding more building prereqs.
The AI quickly starts building offensive units in new cities, but we can slow them down by forcing them to build 1 or 2 buildings first. The Germans in WW2 didn't recruit very many soldiers from conquered territories like France and (parts of) the USSR. Sure there was that Cossack regiment used on the eastern front, and some 2nd line battalions used garrison France, but not a whole lot of offensive potential. The Romans raised few (non civil war temporary) legions from outside of Romanized cities (cities which had achieved citizen status). The reason was these troops were not particularly loyal to Rome, and after enough revolts where the "Roman" auxiliary troops joined the local in rebellion, a policy of stationing these units far away from where they were raised was implemented. Unit loyalty is a neat mod feature going along with rebellions, but is complex. For now it's much simpler to just add some prereqs like having a barracks AND some culture type building (or a lumber mill, since the AI rarely builds those now). Something along those lines. Make those buildings non-conquerable, and there you have it, at least some delay before combat units can be built!
Another thing that may work is to make the static garrisons very powerful, but give them a large city defense malus. In civIII the AI would compare the summation of attack vs defense factors to it's target. In civIV there is no more separate attack/defense, so we have to use the bonuses to get the effect. We can have a 6 combat strength archer with 0 moves and -50% city defense (+ the normal archer bonuses so the actual defense is the same). They would have to be national units so the AI doesn't build hordes of them, but then is unable to attack!
