IME, tall tanks require additional WPG since the light much reach "deeper" into the water to affect plant growth. But it's still possible to grow some of the med light plants, you just won't have huge growth. Certain aquascaping techniques can help, such as sloped or tiered substrate to get the plants closer to the light.
I hear good things about Seachem's Flourish Excel and plan to use it in the near future.
Ya I must be doing something right
. I have all low light plants, java fern, the Cabomba, 2 sword plants and they are all doing really good. I have baby java plants shooting out of my bigger java. The Cabomba has doubled in size, it's not even brown at the substrate. I also have my tank beside a window so I think that helps too