Re: Plants and cycling -
It is true that excess ammonia CAN kill plants. However, the amount we are dosing ~3ppm is not so much that it will kill most plants. The more sensitive plants may suffer.
if you have a lot of plants, then you may want to alter the cycle slightly. Consider dosing only to 1ppm and measure after 24 hours to see how much ammonia the plants are actually using (plants will take ammonia directly from the water column). If it removes all the ammonia, you can boost it to 2ppm the next day. And see how long it takes to return to zero. A lot of plants being added at once can allow you to 'cycle the tank' without actually performing a classic fishless cycle.
So, I guess the big question is this: How many plants do you have and of what type?
Re: the cycle and pH -
Yes, the cycle can affect the pH of the tank during the process. Ultimately, the bacteria require carbon (inorganic) which is generally measured by the kH scale of the tank. The role of the kH is also that it will buffer the water and keep the pH more static. As the cycle continues, ammonia becomes nitrite, and eventually to nitrate (nitric acid) and that will lower the pH (once the amount reaches a certain level such that it will overwhelm the remaining carbonate's buffering capacity). This is referred to as a 'pH crash' and will stall the cycle. The good news is that to correct the issue, all one has to do is a large water change.
This is not a concern (usually) after the cycle is completed, because the routine water changes required for proper husbandry will eliminate that from being possible.
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