The frog per gallon rule doesn't work with dart frogs, most species are quite territorial so no matter the size of tank, on some of the species if you put in more than a pair someone is going to die. Tincs do best in pairs, but auratus and leucomelas would do well in a small group in that size of tank. Waterfalls aren't really a great idea for a beginner, the frogs can drown, and building it usually results in more frustration than it is worth. A drip wall is feasible, if you want to put some extra time and reseach into building. The biggest thing for growing live plants is your soil and the lighting. For my tanks I use double high output t5's with 6500K bulbs. You'll want to have a good drainage layer in your tank, either raised egg crate or hydroton, and ABG is one of the best soils you can use in a terrarium, drains well and doesn't rot your plants.