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.