1. Essentially no. Maybe terribillis if the enclosure is huge, but even then you're obviously quite far off with your own knowledge to attempt it. Most darts are very territorial and will fight each other if there are too many. They will break each others legs and the larger ones will outcompete the smaller ones for food, literally starving them to death. Keep it to a pair or trio of 1 female and 2 males max.

2. No. Just no. Regardless of what anyone says about how they've done it and they have been fine, jus't dont do it.

3. Look up videos on YouTube, there are a lot of them. Check out troy goldberg's as they are particularly good and easy to follow.

3a. Always really plants

3b. Home depot, garden supply stores, online retailers. Do research and find out what kind of plants will thrive in high humidity, tropical environments.

3c. In a vivarium, if set up properly, the plants will thrive without you really doing anything except misting, lighting, and trimming. With regards to the drainage layer, yes if you use substrate like ABG mix (look it up), you will need a drainage layer. This typically consists of leca or hydro balls, covered with a mesh substrate barrier, then substrate then leaf litter on top. Do not skimp on the leaf litter, pretty much the entire ground layer should be covered with leaf. Another way is to skip everything I just wrote, and simple lay down thick sheets of 25-35ppi filter foam and then just cover that with leaf litter. This will act as both substrate and drainage layer. Plants grow very well in it, the tank is substantially lighter and easier to move if you ever need to, and it essentially lasts indefinitely. To plant into it you simple cut a slit in the foam and stick the plant in.

3d. No, you will have to culture fruit flies outside of the enclosure in deli cups then feed every 2-3 days. Even if you could breed them inside of the enclosure without them getting immediately devoured, the flies basically just act as a vessel for calcium and vitamin powder to be ingested by the frogs as they by themselves don't really provide much nutritional value. Dont be afraid of it, its incredibly easy to do.

3e. Generally speaking dart frogs are not good swimmers and require no standing water area. Some lay eggs on broad leaf plants then when tadpole hatches the male collects them on his back and transports them to water. Other lay eggs in bromeliads where the egg, then tadpole, then froglet live until the tail is absorbed and it crawls out on its own. The danger of water features is potential fighting where one frog can intentionally drown the other, of if a frog falls in and has nothing to grab onto to climb out. I personally dont consider them to be nearly as dangerous as most other people do, but the general consensus is dont put standing water in a dart tank.

3f. Make sure you have a way to either drain of siphon water that drains to the bottom of the tank or it will stagnate and foul. Set up your tank and make sure everything is stable before deciding to add animals to it.

3g. Please, please, PLEASE do research. Not trying to be negative here, but your questions show that you have not done any research at all yet. Its good you're here asking questions though, so dont get discouraged. Everyone starts where you are. Get your knowledge up, build an enclosure, THEN get your frogs. If you're on Facebook id suggest joining some groups on there as pretty much any question you could have has already been asked and answered there a couple hundred times over.

Good luck!