i would also like to welcome you to yorkie talk! congrats on your new little rescued baby! what is his name? how old is he or estimated to be? thank you for taking him in!
my pixie who is almost 2 now was originally born in a puppy mill then was in a humane society before i adopted her. when i brought her home at 4 1/2 months old and weighing only 2.0 pounds she had fleas, tapeworm, kennel cough, coccidia, and giardia. we got her on meds for everything, gave her several small meals throughout the day, and lots of TLC and she was all better in no time. we postponed getting her vaccines until all else had cleared up. the kind of food really just depends on what works well for your little one and what he will eat. i will say that yorkies are prone to pancreatitis (i have personally been through this 3 times with 2 different yorkies) and while i know you want to add some weight on him, you should probably not give anything too high in fat content because most yorkies have very sensitive systems. you don't want to give too much too fast until his body can adjust to eating regularly. my 2 girls do great on blue buffalo, while others have had problems with it. wellness or natural balance are just a couple of other good brands i considered.
i would also like to inform you about several illnesses and even deaths caused by chicken jerky treats that are sold pretty much everywhere. the fda has sent out warnings but no recalls have been placed because it is unknown what is in these things that makes dogs sick but they are directly linked to so many sad endings. here's a link to read about it:
FDA Continues to Caution Dog Owners About Chicken Jerky Products
we really do wish you all the best with your new little guy and would love to hear more about him so please keep us updated. yorkie talk is a truly wonderful place and is full of information that will help you so i am very glad you found us!!! hugs!