I too have had good luck with Amazon Swords with a low light tank. My 55gal tank has just two 40watt fluorescent tubes for 1.45 watts per gallon. One tube does have glass mirror reflector. The tank has only natural uncoated gravel, with no CO2 injection, and I use no fertilizer.
One of my amazon swords completely fills the left third of my tank, top to bottom, and front to back. Granted, its taken about 6 months to do this, but I prefer slow growth plants anyway. I'm not sure, but I think mine is also a melon sword. Perhaps this particular type of sword does better than most in a low light tank.
My other plants in this tank, also do very well. My tank is as lush ( but not as well groomed ) as any tank I've seen. I have say that my plants are all easy plants to grow. As well as swords, I have anacharis, wisteria, and anubias nana.
I've tried more light on this tank, but all I ended up with is a lot of algae. So for me at least, 1.5 watts per gallon seems to be about right.
I am certainly no expert on planted tanks. What I did was to buy an plant assortment package that you sometimes see on the web. I simply kept everything that thrived, and got rid of the ones that did poorly.