I am not a vet, but...
Rimadyl is an NSAID, and it's also a big no-no to give steroids and an NSAID together, which can cause bleeding an ulceration of the stomach. There is supposed to be a "washout out", a waiting period between giving these two types of meds, something like 10-14 days.
I have no clue about dosage.
The vet may have been thinking brain tumor, thus the steroid? Sometimes vestibular disease too.
If your dog was anemic, I would look to that as a clue - I would rule out tick disease, and even do a trial of doxycycline, as the anemia and neuro symptoms. Could point to TBD.
I'm sorry that your little one I having such a tough time