i had roughly the same idea on a smaller scale and asked these questions so have a few answers...
1 - it is neither, its a common idea...darts are jungle creatures so recreating their natural environment will always enhance the keeping of the frogs, plus is a good way of understanding their needs a little better - hides, feeding areas, plants etc
2 - most darts wont go in deep water as far as i know, from what im told, so the pool is more for you than them
conisder using a false bottom in here...build a shelf with glass which is the land area, build a beach with rocks (so the substrate is seperated from water by a beach of rocks, plus if a frog falls in he can get out...no shelves!
3 - i am using cocofibre, but this holds a lot of water so make sure it doesn't wick from the pool...hence the band of rocks
4 - this is fine, but make sure anything you put in the viv is sterilised first - you can boil them on the stove for this...i assume you are using a PVC pipe to create the "river"...you can silicone rocks and pebbles in this.
5 - as stated above, anything that wont soak up or overly dirty the water. might be an idea to add a filter of some kind (mechanical would be best)
6 - go with glass as its stronger...ive heard of plexiglass bending or warping under weight and things shifting.
remember, research the needs of the frogs you are going to keep...consider using real plants in this set up as you can have a self-sustaining mini eco system where you need only to add the frogs food
look on here, vivarium forum and Find Me A Reptile (UK but still very good) for journals of peoples viv builds for ideas!
what are you going to house in this viv?