I know you can use the cat or the dog doses on ferrets, rabbits, etc... just like you can with advantage. So my guess would be that if you can use either on ferrets and ferrets are very sensative it should be fine to use on dogs. But you want to make sure you have enough so for ex. if you have the cat dose for 10lbs, you'd need 6 doses for your 60lb dog.

You're vet may be saying that because she either misunderstood & thought that you were wondering if a 10lb cat dose would work on a 60lb dog? Or maybe they are saying that so they get more money? Or maybe it really won't protect your dog.
Maybe call back & explain further or call another vet?