A puppy that young should prob. be fed three or four small meals a day.
Mixing the food is a good idea, I've never been good with % and ratios so I would say start off with mostly the old food and begin adding the new gradualy. It might take about two weeks to totaly switch. I would NOT reccoment going cold turkey and just switching without a transition period for a puppy that young.
I fed Wonder orijen when she was expecting, and she seemed to like it ok... on the rare occasions she would eat that is. She got really picky the last couple of weeks before she whelped. Anyway, I didn't notice any unpleasant changes in her that I could say were caused by the food. I think it's a good food
