My 3 months old dog food!

[Catherine]

Welcome to the Forum. I moved your post to the dog section. It is a better place for it.
You have some complex  questions and I will leave it to one of our dog people to actually answer you.

Generally though, it should depend on the products themselves. They would tell you how long to use their products before transitioning to solid food.
I would think a puppy needs teeth before it goes on to a puppy diet. 

I know more about feeding baby guinea pigs and they can eat solid food right from birth and yes they are born with teeth.