Anything you put on their body, can be
dangerous or fatal to them. If you want to put something on your
cat, to be safe and run it by your vet first.
As for the fleas, and YES, although your cat is indoors only, she can still have fleas. We bring them into our homes ourselves and then they get our cats. The neck is a famous place for the fleas to hit. The fleas have also attacked my cats neck and she scratched her neck raw, too.