I had to get ride of my single polish because the other girls were picking on her too much. I'm so heartbroken.
My formula for mixing breeds with Polish: Polish are non-combative and lightweight so I would only mix non-combative breeds together like Polish, Sultan, Silkie, Araucana, Ameraucana, Easter Egger, Breda, Cochin, Faverolles, Dominique, and Houdan. These non-combative breeds would rather flee than fight and are not aggressive bullies. Sussex, Brahma, and Jersey Giants are supposedly gentle giants but because of their enormous girth I would not mix them with the lighterweight gentle breeds. Even if a giant is gentle it will still be tempted to bully a smaller bird just because it can - it's a chicken thing!
Putting Polish or even EEs in a mixed heavier dual-purpose flock of assertive breeds is asking for the aggressive breeds to pick on the gentler smaller breeds. There are also some lightweight breeds that are wilder in temperament (i.e. Buttercups, Jaerhons, Fayoumi, Campine, Mediterranean class breeds like Legs, Ancona, etc) and would not mix well with non-combative breeds either.
Most breeds mix well together as chicks but as early as juveniles the aggressive breeds can show signs of aggression on the gentler non-combative breeds. We were lucky to have one White Leghorn that was gentle in the flock for 3 years and then suddenly went ballistic on the gentler flockmates. Another Buff Leghorn went cannibalistic at one year toward the gentler smaller breeds. Both had to be re-homed to halt bullying. One Marans clawed viciously at 18 months old and also had to be re-homed immediately. A colorful egg basket is no longer as important to us as having a non-combative flock. Currently pecking order status is a much subtler occurrence between our remaining gentle breeds.