Load up the editor and check the civ's aggression level. You'll have to turn on custom rules, but you don't need to save it. I think France, India, and the Iroquois are the 'most peacefull.' The Iroquois is kinda funny, cuz all my big battles are from them.
But you can take any ciz and put it's aggression level at the lowest, and this will make them peaceful. Or at least more prone towards it.