I had read that combining different kibble isn't advisable because you'll never really know what ratios of the nutrients your dog is getting.
Mixing different dog food brands will improve my dog's diet
This misconception seems to originate from dog owners unconsciously feeling guilty about the type of food they feed, or those following their own ideas for "improving" their dog's diet without doing any research.
Every brand of dog food follows a specific formulation and nutritional philosophy, developed by the manufacturer. All products are formulated to supply a balanced amount of nutrients in a ration of a certain size (remember kibble size and density vary from brand to brand), based on the body weight of the dog. When mixing different kibbles, instead of getting "the best of both", your dog isn't going to eat enough of either one to get the full benefit of a particular nutritional system designed and researched by a manufacturer. Last but not least, if digestive upset occurs, it's going to take so much longer to figure out what exactly caused it, compared to just eliminating either the commercial food or whatever extras were fed recently. If you want to offer more variety, stick to one line of food of the same brand at a time and rotate between brands every few months. Supplementing the dry food with fresh, unprocessed foods like vegetables, fruit, yogurt, meat or a bit of canned food is also safe and healthy.
The Dog Food Project - Myths about Dog Nutrition
While I think this may be true of lower quality dog food, and especially ones with corn and fillers, I do not agree with this assessment concerning high end foods. Manufactures want you to believe this as they want all of your money for their food. But I also think it would be no problem to rotate the food as well, and I tried that, but that is hard on their digestive system with so much change always going on, so,it really never worked.
I ended up on this path because of all the problems we had with them and their loose stools. We had some serious bouts of loose poo, if you know what I mean, and could not get it settled. We would always end up putting them on boiled chicken and white rice to get them back to normal, along with probiotics, and then slowly try another food to see if they would do good on it. Someone, somewhere, suggested mixing food and slowly changing over brands, so I mixed them and they did great on the mix, but never really good on just one. So I eventually ended up with 3 really good foods - a high end protein, a grain free 80% meat base, and a good quality all around one - all of them without corn and junk like that. They really loved it, and no more problems with loose stools. Jedi never would gain weight, but now does great on the mix. The spectrum of nutrition that is in their food, along with the additives I add from time to time, is second to none in my opinion. I base my judgements on my experience and my experience with this has been great.

Ideas.. Suggestions