This is an additional benefit to mixed flocks that hasn't been mentioned yet, it is much easier to track a sick bird or identify issues with a bird when they are easy to distinguish from the others. I have 7 chickens in my backyard, 2 Black Australorps (super friendly and sooo soft) they lay med-large eggs light brown, 6-7 eggs a week each), 2 Wheaten Marans - a little shy but very good to hold (lay large/Xlarge dark chocolate eggs with speckles, 5-6 eggs a week each), 1 Barred Rock - She's the watchdog for the flock, and a little shy (lays large light brown eggs, 5-6 eggs a week), 2 Cochins, they are like little puppies that want to be held and loved on. They are the quietest of all my birds and always softly chatter to us. They lay med eggs 4-5 eggs a week, creamy pale brown eggs.
They are all different sizes and get along very well. I do notice when foraging, they do tend to stay together in groups by color. My 2 Black Austrolorps with the Black Cochin together, the Barred Rock with the Blue Cochin together, and then the 2 Wheaten Marans are always together. Fortunately, all the birds I picked are friendly, quiet, and not flightly. I've had my chicks for a year now, and my neighbors don't even know I have them (except for the 2 that I told, and they LOVE when I give them fresh eggs)