You should carry on adding ammonia until the day before you're ready to get fish. If you don't keep on adding the ammonia, all those bacteria you've worked so hard to grow will starve to death!
You can do your big water change anytime in the two or three days before you get the fish; the nitrate won't have time to build up too high again then.
No, you don't need to use anything in your tank, besides dechlorinator.