I've only known about plants reducing Nitrates, not Ammonia. In fact, that's the main reason why I went w/ a planted tank. Thanks to someone at the pet store telling me that I only needed to do a water change once a month, my Nitrates reached a whopping 160ppm! How my fishes didn't croak is beyond me!
But since then, I have been maintaining a strict 50% water change once a week and my Nitrates stay around 40ppm. The plants have helped quite a bit in maintaining the level, but IME, they are not lowering the Nitrate level like I had hoped. In short, I wouldn't put too much faith in live plants helping w/ your cycle.