Ammonia, Nitrate harmful to Plant?

No, it would not, but I am not clear as to why you would want to do that. Most people who do a fishless cycle using ammonia don't plan to have a planted tank and the goal is to create a substantial colony of nitrifying bacteria to allow a higher than normal fishload to be placed in the tank without causing problems. When you pack a tank full of plants and then dose ammonia, the plants will be in direct competition with the nitrifying bacteria and will likely consume the ammonia faster than the nitrifying bacteria. The only other problem is that starting a tank and pumping it full of ammonia could be an invitation for algae. I have experimented with both methods of cycling: fully packing a tank with plants and using ammonia to do a fishless cyle. The planted tank method worked better in that the ammonia and nitritie spike only lasted a few days vs 13 days(7 days ammonia spike and 6 days nitrite spike using bacterial addition and established/seeded media filter from another tank) for the fishless cycle using ammonia. That means with the planted tank method fish could be put in sooner without problems and with the ammonia fishless cycling method, you are looking at anywhere from 2-3 weeks or even longer.

This is an excellent article on the relative advantages and disadvantages of different types of cycling.
http://www.aquariumboard.com/forums/articles/26.htm

Never do FC inside a tank though, use a small bucket and place the filter in/outlet in there.

I thought these folks have some common sense that proposed the FC method, but I was wrong. They suggest using your tank.

We want a well established filter, so there is no need to add NH4 to a tank, all you need is a small amount of water and the filter.

Why add all that NO3/NO2 and NH4 to large tank simply to cycle the filter?
You can fill the tank up and not add it, run the filter on a bucket ahead of time even before you buy the tank.

Then there's no wait needed.
You do not even need to test, just add a small amount of garden soil(1 teaspoon), some NH3, wait 3-4 weeks, and you are done.

Or you can use sponges/mulm from established tanks, plants, Zeolite, do water changes, live rock, bacterial starter cultures etc etc..............

I've been really critical on FC since it came about.
But after seeing so many folks fall for it, waste their time, money, test kits etc, seeing plant folks get Green water algae blooms, it really gets old and frustrating to new folks.

The easiest method is a simple routine of water changes done weekly for the first 2 months or so, and adding Zeolite which later turns to bio media. You can use old water from another tank if available to speed things up, stock wisely etc.

FC will not save folks that do not bother to change their water, nor will save folks that over stock their tank.

I think, at least from what I've seen, it causes more issues than it solves, there are many other better methods to cycle a tank than this.

Regards,
Tom Barr