I've heard it is not good for a pups tummy to mix grain-free with grain-in. It doesn't digest the same and it can cause stomach upset. I don't know first hand as I have never tried grain-free. That is just what I've heard...
I order my kibble from k9cuisin.com and I noticed a new food on there the other day called Rotations. It is a rotation diet that gives you three different kibble in one box. Here is the link:
Rotations Pet Products - Dry Puppy Food - Free Shipping
I looked on the company's website and it says they were voted best new food. The ingredients look pretty good, too. Plus, they say you don't need to transition - just go from bag to bag.
If you are looking for a rotation diet, this might be the easiest way to go.