are your measurements in inches or CM? The reason I ask is its harder to heat a large tank and also harder to keep humidity high in a large tank. Correct me if I'm wrong but from whati know, the comfortable temperature range for these frogs is 74-82º F during the day and a slight drop to around 68-72º F at night, and A humidity of 80%. which is also difficult in a large tank.
As for plants I prefer Pothos in my tank as I found they do very well in vivariums.
Check out this website for info on plants that are good for Vivariums.
http://www.pollywogsworldoffrogs.com/Text-html/planting-vivariums.html
Moss isn't required for the BETF, just use terrarium substrate which is coconut ground fine. They come in bricks, just add water. I recently found moss in a package you place in your tank that will come back to life if you keep the humidity high. I'm still experimenting with moss in a large tank. Seems to grow at the bottom where humidity is highest.
Anyways let me know if your tank is in inches or CM and ill give you more advice on heating and humidity.
http://www.frogforum.net/vivarium-terrarium-enclosure-discussion/2497-10-gallon-vertical-dartfrog-terrarium-construction.html
Check this out, I suspect this would work well for BETF, and could be easily adapter into a rain chamber.