Don’t mean to repeat what everyone has said so far, just want to comment so I can follow the thread. I have a Rat Terrier mix and he is a little over 1 year old, and has been on 11+ different kibbles so far. Variety is essential so you can look for good quality protein (from meat or meat meal), and good brands, and rotate flavors and brands. I think I’m not doing it quite perfectly, as I choose a brand, then rotate all flavors I am willing to feed within that brand, then I switch brands. I think you should alternate brands and flavors between bags and go back and forth between brands if need be. Mixing kibble with canned and raw is also a great idea. Definitely avoid the Science Diet and low protein. Good affordable kibbles that I have fed are Victor Grain Free, Earthborn Holistic Grain Free, Vets Choice Holistic Health Extension (protein is below 30% though), and Dr. Tim’s. All have manageable kibble size for small dogs (Dr. Tim’s is the biggest of those listed, and Victor is the smallest). Good luck!
Oh, and when transitioning from one kibble to the next, you can add some plain pumpkin puree or yoghurt/kefir, or canned to make the transition easier on their tummies. For canned I currently use Nutro Ultra and Nutro Max.
