Cornish crosses are more prone to get cold than most chickens due to the lack of feathering. They love sleeping under the lights. ALWAYS keep them separate at night from your regular flock. I can't even imagine giving my birds sour milk- what is the point? What is the benefit? Are you increasing health, or cutting corners on feed to just fill the bellies? What are your temps at outside at night? Do they have proper ventilation? Access to water at night?