Time is the answer for your concern as I assume you have only had the pup for a short time.I predict that within 6 months the older dog will adapt and they will be buddies. I would not try to force the issue and make sure the older dog can be alone when she desires.
Geriatric Vestibular Syndrome (GVS) and or a herniated disc in her her spine are up on the list of possibilities.
GVS is similar to vertigo in people and will usually resolve in 4 to 6 weeks.
We try to discourage clients from declawing but we perform the surgery at 6 months of age.
Its common for Yorkies as well as as all toy breeds to have KNEE problems dislocating.
This can be repaired surgically with an excellent prognosis. When a hip dislocates it stays out of place.
Your lab should have an Xray for bladder stones and a urine culture to rule out infection.
He is probably drinking excessively and the reason should be revealed with some simple blood tests to check his liver and kidney function and diabetes.
I would have him evaluated for thyroid disease, diabetes and kidney disease.with some routine blood tests. This with a thorough exam.Hopefully it is the additional exersize causing the weight loss.
