I think it's a great idea, as long as all of the foods are high quality, and you aren't mixing a poor quality food (ie Pedigree) with a high quality food (ie Innova) - which, all of those foods are good quality. I've heard that slowly mixing in a variety of high quality kibbles is great for dogs, as well as mixing in veggies & meats & yogurt and whatever other wholesome supplements. Of course, for people with dogs with allergies, it's best to stick to a kibble that doesn't trigger any bad reactions.

Personally, I would like to mix a slightly cheaper food (like Nutro) with Gonzo's Evo, but he wouldn't have it. He has very picky taste in kibble now, as he LOVES Evo, Innova, Wellness & Solid Gold but absolutely nothing less. Some one got him Iams sauce stuff for X-mas, and when I poured it on his Evo he refused to eat it until I refilled the bowl with plain kibble. I guess he knows lower-grade ingredients >_<