IME you can mix pretty much anything as long as there is enough space. I've been mixing large breed laying hens, meat birds, banties, all sizes of ducks, etc into one flock & have never had any issues. I've had a few breeds that were supposed to be bullies but they never caused any issues...if they decided to charge someone that hen would just go forage in a different part of the yard with her buddies. I don't generally handle my birds (except for health checks) & even the flightiest will come running up to me to see what kitchen scraps I've got. I'm mostly interested in egg production, predator smarts, foraging ability, & having a wide variety of breeds because I like the way a mixed flock looks. (Disclaimer: I've never done Polish because they just look like they'd be a buffet to predators to me & I don't want to train the hawks in the area that chickens are easy picking (they stick to the squirrels now).)