Hi,
1) The dranage substrate is used as a place for extra water to drain into so that the soil doesn't become saturated with water. You will be spraying the terrarium with water at least once a day and over time this builds up in the bottom of the tank.
2) A false-bottom is like raising the entire insides of the terrarium up an inch or two which forms a place for water to collect below. This is just another way to prevent the soil from becoming soaked with water and turning into mud. The pros to using a false-bottom are that it is lighter, it makes it easier to break down and clean out the terrarium and it can be cheaper. It doesn't look as nice though.
3) There is a wide variety of plants availible. If you are looking for plants that grow well and are hard to kill give pothos and philodendrons a shot. Once you start keeping dart frogs, chances are you will start to get interested in plants and want to learn more about them and try keeping more difficult species.
4) Use a substrate that holds moisture and won't spoil or go bad easily. Gravel can be used in part of the terrarium but shouldn't be the only substrate in the cage. I have always used a soil mixture.
5) Everyone will have their own oppinion about this. Dendrobates azureus, leucomelas and tinctorius are all large, hardy and bold frogs that are fun to keep.
6) There are a few different ways to construct ponds in terrariums. I think the easiest way is to slope part of the substrate down and then fill the tank with an inch or so of water. This is another reason you might want to use a dranage substrate like gravel. The gravel can be shaped so that in one corner it slopes down to about a quarter inch. The terrarium is then filled with enough water to create a pond. Its kind of hard to describe, have a look at http://www.amphibiancare.com/frogs/images/terrarium44.jpg for a picture. Good luck!
-----
Devin
devin@amphibiancare.com
www.amphibiancare.com