I realize this is an older thread, but thought I'd add my two-cents' worth. We got Bonnie, our Jack Russell when she was only 3 weeks old. She was basically healthy and didn't have any medical problems, but was the runt of the litter. We started her on special puppy formula (available at most pet stores) using a bottle similar to a baby bottle only with a larger hole in the nipple. Feeding was every 2-3 hours at first which meant a lot of sleepless nights for me.
At around 5 weeks I started giving her just a little baby food in addition to the puppy milk. (I don't have a proper food processor to do it myself.) I doubt you folks have the same brands as I do here in Germany but I looked for bio-products with no preservatives or artificial colouring/chemicals, and pure meat only - chicken and turkey puree to start, then beef. Nothing chunky, and nothing that had any vegetables or other stuff mixed in.
I used a teaspoon and got her used to licking it. Gradually I got her used to eating from a dish and phased out the milk formula. At 7 weeks I began to add a little commercial puppy food. Then I finally switched completely over to high quality commercial puppy food at around 9 weeks. Now she's a completely healthy 14 month old and eating "young adult" dog food.