Hello, I'm Dr. Deb.
I recently came online and see that your question about Taz hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
I can understand your concern for Taz; I'd be a little worried about her as well if she were my cat.
I suppose it's possible that if Taz was weaker because she hasn't eaten well for a while, her balance issues might make sense; however, given her age, there may be other explanations as well. She could be developing arthritis issues in her hips, for example.
Her lack of interest in the new food may explained by the fact that she doesn't like the taste of it but, again, there could be other reasons why she didn't eat it.
2. Kidney issues
3. Problems with the liver/gallbladder
4. Infectious disease such as Toxoplasmosis especially if she's running a mild fever.
If she continues to have balance issues, then a vet visit may be prudent. If she continues to eat more each day and continues to otherwise act normal, then a vet visit is still not a bad thing to consider but might be less immediate.
If blood work hasn't been done on her in a while, if nothing else it would be good to get a baseline and to rule out any other issues (as much as possible anyway).
I hope this helps; again, my apologies for the delayed reply. Deb