I believe there are a few members here who do feed a mixture of kibbles. I don't think there is anything wrong with that, though it could make it more difficult to identify the culprit in the event something doesn't agree with your dog.
I do think that rotating kibbles is a good idea - no one single food has it "all" and I believe variety is a good thing, especially when it comes to protein sources. LOL - dogs in a natural environment would never have eaten 'only chicken', or 'only lamb' for their whole lives. You'd never find a nutritionist recommending a person ate 'only chicken' as a protein source either.
Some people who feed a mixture of kibbles do so in order to provide variety. And I think that that is better than feeding a single kibble for an extended period. But variety isn't just about getting a mixture of different things, its about not eating the same thing every single day
So if you do choose to mix kibbles, it is still a good idea to vary that mix from time to time. If you fed (for example) a mixture of Canidae and Wellness for a couple of months, then it might be good to substitute the Wellness in your mix with Chicken Soup (so now you've got a Canidae/Chicken Soup mix) for the next two or three months.