Please use Advantage on your cat. It can be bought over the counter now. Stay away from sprays and other over the counters...Frontline is good too.
Yes it is safe to use Advantage after neutering. Many organizations who neuter ferals also give a dose of advantage while the cat is still asleep from the neuter.
But please, use either of the above. Keep your cat safe.
Please call your vets and ask for their flea products they are safe with the correct doze. My vets offer advantage and revolution. They also can cause worms so I'd suggest a worm medication also or revolution because it deworms.
please don't use any over the counter medications. My vet said no medication is better then over the counter.