Dog food is safe for the puppy to eat. However, it might lack some important nutrients that are required by the puppy for its growth. In addition, when feeding puppies, you need to pay attention to protein minimums and fiber maximums. For more information, check, http://www.dogfoodadvisor.com/best-dog-foods/best-puppy-foods/.
Yes, but puppy food has different vitamins and nutrients in it to help your puppy grow and help with brain development. So, if you can, you should feed your puppy a good puppy food while he is growing, usually for about the first year of his life. Feeding him dog food if you run out of puppy food wont hurt him though.