To me the most homogeneous societies seem to be frontiersmen living in extremely harsh places. The Sami, Pygmy tribes and Inuits we're left alone for most of history. Nobody can fight off immigration indefinitely if there is some reason for them to come.
The most violent civilizations in history didn't seem to care who they oppressed. Roman Empire, Mongol empire and British empire had no problems taking different ethnicities as subjects. Some of them even had somewhat varied ethnic makeup in the power-base of the empire. For example Romans had Latins, Etruscan people and Greek city states assimilated into Rome proper.
This idea that ethnic makeup dictates success and violence of society is essentially racist. It doesn't really matter if you advocate a mix of ethnicity, you are still trying to sell policy with race. The only non-racist stance is to assume that ethnicity and success have no correlation what so ever.