You can add plants before the pond is cycled. Have you ever had duckweed before? It is a great nitrate remover, but multiplies very rapidly and will cover the whole surface of your pond in a very short period of time. Once it's it in there you may never be able to get it completely out again. If you goal is to be able to have any kind of opportunity to view the fish in this pond I would avoid duckweed like the plague. There's lots of other options that are easier to remove as they multiply. Just a thought...