Well I for one don't have many slaves in one city. I use the fact that slaves require military units to gaurd them. I move my units out of all my cities next the unit and leave units in the city where I want the slave. This way I can control where the slave goes, and when I have to many slaves in one city. Sometimes though I get a slave when my units are attacked by barbarians, but I usually attack them before they attack me. In this way I have almost the same number of slaves in my cities. Unfortunetely the AI doesn't do this. I for one think there isn't enough slaving in the game, I have never been able to get my slave population above 15 percent of my population. I think that the Idea of the slaves reproducing as time passes is a good Idea. it would most definetly increase the percent of slaves.