I would wait until the dog is at a healthy weight before you start doing work on the RG. At this point, when he's still starving, I think it's a little unrealistic to try to make significant behavioral changes, and they may diminish anyway when he reaches a healthy weight. Training cannot mitigate illness, and right now I think you should treat him as a sick animal.
I think practicing self-control exercises is a good idea, but it's going to take a few weeks for the dogs to sort themselves out, and for you to start seeing this little guy's true personality. I think it's totally understandable for an animal who was living on the streets and starving that has suddenly been given all of this safety and abundance to be possessive of it. It may be a product of the situation, not really who he fundamentally is.
Make time for all of the dogs individually for training, play, and affection. Make sure your sheltie has a chill-out space where they can get away from the new addition. While you are in training, I would also remove temptation and not let the dogs have access to any objects the pom will guard without supervision. That way, when you want to start your counter-conditioning program, you will have total control of the situation and not set the pom up to fail by letting him repeat the behavior unchecked.
In case you haven't found it in your travels, this is one of the best books on RG out there.
http://www.dogwise.com/itemdetails.cfm?id=DTB740EBK