My definition of fin rot is fins getting shorter with nothing nipping them.
Not sure about the nutrafin cycle. I prefer to keep "cycled" extra filters running that I can move to hosp. as needed. Some of the "seeding" products can be high in ammonia or can have so many bacteria they deplete the oxygen in the water. I would use the Cycle in the tank, but a few days before adding a fish. Trying to "cycle with fish" in a small tank with an already ill fish is asking for trouble. Better to assume an "uncycled" tank, use a product that "detoxifies ammonia and nitrite" chemically and change water everyday. IME, extremely clean water is the best med for finrot anyway.