Well, that Royal Canin does contain corn and a lot of dogs are sensitive to it. It could be just about anything, though, and it's frustrating.
The solid gold is pretty large kibble. I've never seen the chicken soup, so i have no idea about that, sorry
Mine get a mixture of Solid Gold, Blue Buffalo, Castor and Pollux, and Wellness
Ended the pickiness, I tell ya!
Both of mine are puppies, though, I can't imagine having to keep their food seperate, that must be hard.