I have used skinsosoft...on my dogs. My neice uses it on her horses. But I am not sure about a cat.
However I can tell you this..get some brewer's yeast powder.
Sprinkle on the cat's food, not much just a little at a time.
It will not happen overnight, but it will work. What the yeast does is it changes the taste of the cat's skin to something fleas do not like. No I do not know what the taste changes to, but it really does work. Hope this helps. Patricia Chapman