I have used Hill's ID many times with great success. I also use Purina EN instead of the ID at times. They are pretty much the same food.
My Rookie recently was hospitalized for tests due to GI issues. His xrays were quite scary .. super enlarged liver and so they did an ultrasound which suggested that he could have a number of things (including cancer which scared the heck out of me). We ended up doing an endoscopy on him and he has lymphangectasia ... for now he is on Purina EN and doing well. He is not yet losing protein so they didn't want him on a super low fat diet.
Anyway......my point to all of this is that Hills ID and Purina EN are great RX foods!
I had a pup many years ago who was on the EN for the last 4 years of his life and he lived to be around 16.
Also, the vets must warn you about cancer because it is always a possibility, but yorkies are SO prone to gastro intestinal issues that I would try hard to think that it is probably one of them. Believe me, there is a greater chance of some gastro issue than cancer in a yorkie! Often we spend so much time worrying about things that don't come to pass.
Good luck and please keep us posted!