You haven't mentioned how much light you are using. With higher light the plants grow faster, using up ammonia and other nutrients faster. With typical "natural tank" low light, the plants grow very slowly, so don't use nearly as much nutrients, including ammonia. If that is what you have, I would add a few fish every week until I reached my stocking level. Unless you plan to quarantine each batch of fish before adding them, the problem of one sick fish infecting the whole tank is the same however you do it.