It would definitley make sense, I like the idea. Milita could have the most positive effect, regular soldiers a normal positive effect, and foreign mercenaries a negative effect. I don't think that the presence of national troops would really add to unrest, unless the city is ethnically foreign to the troops. It was a time of nationalism, and the average citizen would probably take a lot of pride in seeing their military.
I like the idea from Civ4 how you have extra unhappiness if you have no troops there. The efficacy with which troops pacify the population should be proportional to the size of the largest army, and the importance [maybe tech level * population] of the city. I assume that regular police forces had been set up by that point in time, which is why I don't think troops help much except during a war.
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