The different breeds don't matter. You can see in my signature what breeds I raised together. What is important is getting them about the same age. More mature chicks are more likely (not an absolutely, happens every time, disaster is sure to follow, will not fail, just more likely) to pick on younger chicks, sometimes causing injury or even fatalities. Age is more important than size as it is more a matter of maturity than strength. Some people mix chicks of very different ages and don't have problems while others try it and wish they had not. A lot depends on the personalityof the individual chick.