I'm using Petco mice to breed, they do a real good job at sexing mice. Plus it seems they get them pretty young so if you throw a bunch together they will do alright. Seems like Petco also has a wide variety of colors, you can get all white or mix it up. I got some really fancy ones, some are almost too fancy to feed to my snakes LOLOL. Adult mature male mice tend to fight, I've seen some really nasty wounds on some adult mice in my local pet stores. The weird thing about Petco is that they keep a mouse tank full of male mice, apparently they don't fight if there are all males in the tank with no females. Just like the chickens I used to have, put one rooster with a bunch of hens and they do fine. Throw in another rooster or two and the roosters will fight to the death. Get rid of all the hens and have a bunch of roosters running around and they are perfect gentlemen LOL.