Rotating Puppy food?

I've read several of the threads dealing with rotating food (kibble). Whereas before, I assumed staying with one food would do, I can now see why rotating can be beneficial ( avoiding dev. allegies, supplying different nutrients, etc ). My question is can rotating food be started as a puppy (12 weeks)? Are there any drawbacks or health issues to avoid doing this as such a young age. It would seem logical that a wider range of nutrients/vitamins would benefit a pup still going through growth spurts; but I dont want to put any undue pressure on a young digestive tract either.

On the same topic, I've read several posts about supplementing kibble here. People include plain yogurt, eggs (hard boiled and raw), vegetables, sardines, etc. Again, is 12 weeks too young to start a pup on some of these? I want her to have every developmental advantage I can give her, but dont want to start too much too soon. Any advice would be appreciated.