I also have a mixed flock of 10, with no more than three of any one breed. So far, life is pretty harmonious, although the newest members of the flock always have a bit of adjustment period where they get chased away from the feeder and have to figure out where to roost for the night, as the established chickens won't give up their favorite places on the roost.
That said, I have read articles, both here and on chicken breed websites which suggest that certain breeds of chicken are more aggressive and certain breeds are more docile than other breeds, and that you can get in trouble with throwing a few of the more docile birds in with the more aggressive ones, because they will be harassed and picked on more than you'd want to allow. I do remember a warning,for example, that if you put a a one or two Polish in with a flock of standard birds, the funky head-dress will get too much attention and feather pulling for the poor Polish's good.
So, with both things in mind, my recommendation is to introduce the new birds (more than one so they have a friend) starting with a fence between them. Once you decide to put the new in with the old, be sure there is room for the new ones to run away and hide if necessary, and consider adding additional feeding and watering stations, since the old ones may drive the new ones away. Keep a closer eye on them for the first few days if the harassment seems excessive. I imagine you'll be fine, but it pays to be extra careful in the beginning.