Just to clarify, you are mixing sunflower seed, mealworms, scratch and oats with your feed?Yes i mix it with the feed!
All those things are considered treats. I know they love that stuff, but what happens is your little chickies will pick out all those goodies and not eat the nutritionally balance feed that has the essential vitamins/minerals and protein.
You mention they are fat? Are they really at 2-3 months old? Extra fat can cause reproductive problems and liver disease as well.
Cut down your treats to no more than 5-10% of their daily intake, they need to be eating their poultry feed for the majority of the day.
I would still treat with the Corid since you are seeing bloody poop. If you see any more, please get a picture.