Welcome to BYC. Glad you decided to our flock. You can definitely mix different breeds of chickens as long as they are of similar size and not overcrowded. I would avoid mixing standard sized breeds with bantams or the crested breeds. If you live in an area with really cold winter temperatures as they are not as cold hardy as the dual purpose breeds and their combs are definitely subject to frostbite (often even vaseline won't help with Leghorn combs). If you are thinking of going with Leghorns for their excellent egg production, I would recommend going with Black Sex Links (Black Stars), which are egg laying machines, instead. Not only are they friendlier than Leghorns (which are flighty and high strung), but they are more persistent layers in really cold winter weather than Leghorns. I've raised Black Sex Links for years (along with dozens of other breeds and hybrids, including Leghorns), and they have been my best layers, consistently churning out more than 300 eggs per hen per year, even in areas with really cold winters. Please feel free to ask any questions you may have. We are here to help in any way we can. Whatever breeds or hybrids you decide to get, good luck with your flock.