Cats are much more sensitive to meds than dogs of ANY size. You can't even give kittens the same replacement milk that you do pups. There is an 'everything' replacement milk that can be used for just about everything - pups, fowls, calves, etc. - EXCEPT kittens. You can ask anyone who does rescue - 95% of all orphan kittens die while 95% of all orphan puppies live. They are just too delicate. EVERYTHING is metabolized differently. Dogs are bascially the same - the difference in doses being because of weight. Cats are not just different because of their size - their metabolic makeup is extremely different. Some vets don't even treat cats. And there are some vets that ONLY treat cats. They are extremely specialized almost as much as birds.
PLEASE at least call the vet or the shelter. I would hate for you to try to do the cheap and easy and lose your furbaby. I have had it happen too many times to people that I know. You don't want to live with a life time of 'what if's' for what is really a fairly small amount of money.