If the poops are formed but just soft I'd give it a little more time and see if things settle out. If the poops are super sloppy/liquidy or they don't improve over time, it's worth consulting a vet for an exam and a fecal.
For portion sizes, I tend to go by dog's condition. If they're maintaining a healthy weight, not acting desperate for food (though it's worth mentioning some dogs/breeds will do this no matter how much they're fed), and - in the case of puppies - growing well, I'm happy. I'm inclined to say stick with one food and avoid throwing lots of different things at him unless there's a serious issue, at least until your puppy has had time to settle in and get used to your household, because this is a stressful time for puppies and messing with their diet unnecessarily can easily make it moreso. Stress can cause softer poos too, so time may well resolve things, but again, don't mess around if his poops are really sloppy or diarrhea. Like infant humans, baby puppies can dehydrate very quickly if they have diarrhea and/or vomiting.
