I haven't read birdseed labels recently. What percent calcium does it usually have?
That's another bad point about this recipe: we don't really know if the scratch is 1/6 or 1/4 or what of the feed, because there are no "parts" listed for the cat food and the rabbit food. They could be almost nothing, or they could be in even larger amounts than the scratch or birdseed or layer pellets, and it just does not say.
Given that the list says "every other day" for the cat food, I'll bet the scratch is more than 1/6 of the diet.
Rabbit food is mostly hay, ground up and squished together into pellets. A little bit is not going to hurt chickens, but it will not be any better for them than providing a bale of hay for them to peck through: a little bit of variety in the diet, but not a big proportion of what they need in the day.