I've never fed Royal Canin...I fed Eukanuba until I found out what was in it, and then switched to Blue Buffalo, then to homecooking food. I still give them a little BB kibble from time to time.
You can research your choice in food, as well as kinds you're thinking of switching to at
www.dogfoodanalysis.com
Most people on here recommend a 4-5 star food for yorkies. 6 star foods are for large working dogs, and probably are too much for yorkies.
If you'd like more information on homecooking-do a search in this forum section-there's TONS of information on here. And, of course, if you can't find what you're looking for, please ask! Us homecookers are happy to help you get started!