If one have live plants that consume ammonium, there is less of a need for bacteria. The goal of cycling a tank is to handle the wastes the tank produces. The first step in this is the creation of the ammonia (NH3 and NH4) we do not want. If the bacteria handle these they two step it to nitrate which we remove via water changes, However, live plants will also consume ammonium (NH4). The difference is the plants use the nitrogen in the ammonia and they do not create nitrite or nitrate. In fact they will also consume nitrate.
Plants can uptake ammonium (NH4) and a faster rate than bacteria take up ammonia (NH3). Bear in mind in water there will be both NH3 and NH4 (but more of the later than the former). So when the plants reduce the total ammonia, there is less left that require bacteria to handle. Think of it this way. If one is cycling two identical tanks except they are adding 1 ppm of ammonia to one tank but 3 ppm to the second tank, which one will have more bacteria? Well it is the same when the plants are involved,
The final piece of this puzzle falls into place when one realizes that plants consume the NH4 but the bacteria take up NH3. Normally there is mostly NH4 in our tanks. However, there will always be a balance between the two forms based on the pH and temp. of the water. This means there will always be both forms. Even if one plants very heavily, there will still be some amount of bacteria present to take up NH3. And in tanks with only a few plants there will be a lot more bacteria.
In the end, it doesn't matter from a making a tank safe perspective what the balance is between plants and bacteria doing the work, only that it be done. The other thing to realize is that plant roots and leaves are covered in bacteria. So when one adds plants to a tank, one is also adding bacteria as well.
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