Never. He or she could get sick.
You shouldstart feeding your puppy solid food when it is 6 weeks or older. You should start out with soft food and soon switch to hard because it is better for them.
Yes, you can use Enfamil AR Formula on puppies, either for supplemental feeding to give the mother dog a break, or if you're hand feeding. Keep in mind that around age 3-4 puppies start to get weaned from their liquid diet onto a semi-solid mush. After 5 weeks of age, the puppy should no longer get formula or a milk replacer.
You would have to bottle feed the puppy until it is old enough to wean and get on solid food. If it is two weeks old and healthy that could happen fairly soon. There are puppy milk formulas available that you can feed or you can make your own. There are several recipes on line if you search "bottle feeding puppies". Bottle feeding infant puppies can be very difficult but if a puppy is already two weeks old it should be easier.
Puppies should stay with the mother dog for 6 to 8 weeks. The pups should be weaned by the end of 8 weeks and eating solid food. Then they can be adopted.
puppies can eat solid food around 8-12 weeks after they are weaned
The puppy needs to eat at least 2/3 times a day you should feed them everyday A good place to start is with the recommended amount shown on the particular food you are feeding and adjust as needed from there. Twice a day feedings are typically enough for puppies once they are eating solid food. It will be pretty hard to feed them too much at six weeks since they are burning a lot of calories growing.
For the first few weeks, puppies are nursed on milk produced by the mother dog. As the pups develop they need to start transitioning to solid food. If assistance from the breeder is not available to produce gruel for the puppies to eat, the mother dog will eat and partially digest food which she then vomits for the puppies to eat. This sustains the puppies while they practice eating solid food until they are able to digest it themselves.
Dogs can get stressed out if their is too much commotion or if the puppies are ill. Likely, you are handling the puppies too often or their is too much noice. Try moving the puppies to a darker and quieter location and feed the mother next to the puppies. She may take them back
...How Long* do I wait...
When the puppies start eating solid food, they suckle less and the mother's milk will dry up.
Can they - yes, but puppies should stay with mom and the litter until 8 weeks of age. Usually as they start on solid foods, mom comes around less and less but it is important to allow them the full 8 weeks or longer to be together. This is a critical learning stage for the puppies to develop dog social skills that will aid them later in life. It is a proven fact that puppies taken earlier have a higher percentage of developing behaviorial issues as they grow up.
Introducing solid foods can increase the risk of gastrointestinal infections and allergies. 6 months is when you should be weaning, see here: healthvisitors.com/hv/5/125