Well if your ammonia was at 2ppm (are you certain it was at 2ppm?) and now is at 0 I would say that the rock you added was already fully cycled and you are now safe to add livestock. It would be a good idea to make a major water change first to help bring down the nitrates, but the ammonia and nitrites at zero indicate the bacteria can process fish waste.
2ppm to zero in 24 hours is what people who "stress test" their biofilters by pouring ammonia in the tank aim for. They say when your bacteria can do that the tank is ready. So it's not bonkers that your tank could do it. but I think also the really ammonia smell should be gone, and I like to feed for a week without any ammonia showing, personally.