This is one of the most persistent myths of poultry keeping there is in my opinion.
It's a misunderstanding of what a "hot feed" means. Corn is a rich source of easily digested carbohydrates which in the parlance of feed composition makes it "hot." What it does NOT do is make your birds hot. There is a certain amount of heat liberated during the digestive process but this is normal and it occurs with any feed component, particularly the fibrous ones. It's normal, natural, and your birds are quite capable of adjusting their internal thermostats to keep their body temperatures where it should be so far as the heat of digestion is concerned.
Now whether you want to feed scratch to your birds depends on what else they are eating. If they are free ranging and finding plenty of bugs and other good things to eat you can throw down the scratch with a free hand. They'll generally balance things out on their own. If your birds are confined to a yard or otherwise restricted from foraging for themselves due to cold weather, drought, or whatnot then restrict the scratch to about a half a handful per bird per day if their main feed is the typical 15-16% protein content of most layer feeds. Feed a lot of scratch in that case would only serve to dilute their protein intake which in turn would lower their egg production.