Don't forget that when you mix ANY thing else with your chicken starter/grower, then you are cutting your protein (unless it's something like dairy or meat!)
Therefore, if you have a 20% starter/grower protein, and you're mixing it with cracked corn (9% protein) 50/50 - your birds are now receiving 9.5% protein - NOT NEARLY enough.
Plus, carbohydrate AND protein both yield 4 calories per gram. A calorie is actually a yield of heat produced. (Don't worry about it - it's not relevant for cold weather).
Therefore - your birds are protein-deprived. Supplement fast with dairy/cat food, etc.
THEN put them on a high-protein grower ration. It's fine to supplement with fresh greens, fruit etc. This will provide extra vitamins and minerals. In fact it's a really good thing.
HOWEVER, 95% of your birds diet should come from a high quality balanced food specific to their stage of growth ...starter, grower, layer, etc.
Just imagine - replacing 50% of your CHILD'S diet with popcorn!!!!!
Although your chicken's problems may NOT only be nutrition related - it's a good place to start. PLUS - if nutrition is compromised, then so is the immune system.
What a shame you got such bad advice from the feed store. They really should know better.