I have 13 breeds (and counting...) and everyone seems to do fine together - bantam vs standard, etc. I tried to pick breeds that were noted to be fairly "docile" and not too flighty. Ironically, my biggest bully is my blue silkie chick - who is not being very welcoming to my latest - rock barred, Delaware and GLW.
I also have birds that are all within 3 months of each other's age - all young birds. I figured introductions while everyone is still young would have a likelier/easier success rate that introducing adults.