When deciding which breeds of chicks to order for your flock, it is good to consider the temperaments of the different breeds. For example, the buff orpingtons and wyandottes are known to be a bit more docile than the more aggressive breeds like the rhode island reds or old english game birds. Keep in mind that mixing these breeds may make pecking order (the time that those birds assert their dominance over one another) a bit more difficult for the docile breeds.
That being said, these birds will have a much easier time getting used to one another if they were raised at the same time since they will be the same age and size, and a bit more used to one another. Ultimately, the birds will get along easier if they are all the same breed, but it is still ok to mix them.