I don't think so. Otherwise the socialization is a bit stilted, and you run the risk of an unexpected situation out in public. It's like training your dog away from all distractions. He'll behave perfectly in that environment, but the moment he's outside, you'd think he'd never heard a command before. I think of course that the dogs should be closely monitored in a mixed class...some dogs do fine with dogs their own size but will try to eat a small dog, or vice versa. We were fostering a rescue Airedale at work, and while he got along fine with the other big dogs, anything shih tzu size or smaller gave him this "i'm hungry" gleam in his eyes. I've seen it in other dogs as well.