I know the dark substrate makes them look fabulous - I despise my remaining white rocks when they pop up to the surface. Even just a sprinkle of them makes the neons look washed out. (I remove them every few water changes, maybe one day there won't be any left. *lol*)
They like driftwood I hear (which also helps to keep my pH lower, 7.0-7.2 when the other tank is more like 7.4 to 7.6), and I've heard some reports that adding Black Water Extract can even act as a deterrant to NTD. I'm not sure about that, but I'll be trying it again once my infected fish finally go.
You can use floating plants (real or fake) to dim a portion of the brighter light, in areas where you might not have a higher-required plant.
I'm sure that they can get by without real plants, as long as it appears well planted and safe. I think the real problem is finding a good stock. Don't get them ANYWHERE where they don't offer a 14 Day guarantee and always quarantine new fish, including the first batch. (Sure wish I had.) And any fish you introduce after that. (Or buy them online, which is what I would do if I was going to buy such a large school of them, liveaquaria.com.)
They also need a pretty delayed acclimation, the one-hour deal to float and mix small portions (I do 1/3 cup), into their bag every 5 minutes.
I've been doing extensive research on NTD as well, and some people say that they feed the Jungle Anti-Parasite food to their neons once a week.
Also, it's possible in a tank that size, and with no other fish, they may not have any reason to school, and may spend most of their time scattered around. (To some extent, they do tend to pick little territories, at least mine have.) I LOVED my neons, but my centerpiece fish too, was great, he was probably my favorite in the end, and when he went, my WHOLE tank looked miserably empty, even though all my Neons were still there. (But I can also understand how the species only Bio-Tope is a neat concept.)
I would think 15-20 would work, depending on whether it's High or Long. They do like to swim a little bit, although not quite like danios or minnows.
Also, I've heard that they don't really thrive in high temperatures - my heater won't go any lower than 80 and I feel that was a mistake on my part and might have led to them contracting illnesses. I'm going to downgrade my heater for my second attempt, so that I can keep closer to 75-78 max. (My Quarantine Tank is around 76-78 and I haven't had any outbreaks of anything so far. I plan to quarantine the new neons probably for a month. It took about 3 weeks for my first potential NTD symptom to show up.)