My mind is swimming with all this water chemistry stuff that I'm doing my best to assimilate....but how do plants affect the KH stability of our water? If I have to use buffers to control my KH/PH (I do), is there anything I can do or should be doing with the plants to make sure they don't make it any worse?

Plants by them selves have little impact on Kh. In my experience, the only thing with plants that can make Kh drop signifigantly is if you are using CO2 injections. This would usually happen if too much CO2 is being injected for the number of plants and the amount of light that you have. CO2 can cause a gradual decrease in Ph and Kh since carbon dioxide dissolved in water creates a mild acid. But if the amount of co2 is balanced with the number/density of plants as well as lighting, Ph and Kh drop are not an issue

What's the best way to figure CO2 in the water? Isn't there a chart with KH/PH readings that tell that? I gotta find that again.
Thanks BR!

here u go hun.

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm
I used to use this all the time