At 6 weeks they are only just completely weaned. We collected Flo at 8 weeks and the breeder had been giving her puppy kibble softened with warm goats milk (which is the closest to mums milk) and I think she had only just stopped feeding from mum. When I brought her home I continued with the same kibble the breeder had used then gradually switched her to James Wellbeloved and sometimes used warm goats milk with the kibble to tempt her to eat as she was a reluctant eater.
Agree fully. DO NOT take home a pup at only six weeks. If a breeder will allow it, their scum to avoid. The dogs don't matter, the pay checks do. If they do seem to care, then there ignorant which is just as bad! If they don't know about weaning right, I truly don't believe they care (or even know!) about health, proper care, shots, worming ect.
As for food...
Go here. http://dogfoodanalysis.com/ Get a 5 or 6 star food. Look up the site for the individual food, most have locators. I feed Tasteofthewild. Yes, some of the higher foods can be expensive compared to the sawdust in a bag often seen in stores. You'll save in the long run when your dog has less health problems due to proper nutrition though.
Do NOT believe you vet if they say that iams, science diet, ect are good foods. Their trash in a bag. Look it up before you believe anything about food from them!
Go here. http://www.truthaboutpetfood.com/ This lady is a dream come true for good dog owners. She warns you about problems, keeps you up to date on dog news, and has lots of dog food and ingredient facts.
For care and info on how to raise, train, live with and understand what a dog really is: