A pair of ranitomeya would probably do fine in there, assuming you landscaped it properly to their needs. They are generally arboreal so a vertical orietated cage would suit them best. Check out the care sheets here on the forum : http://www.dendroboard.com/forum/car...re-sheets.html
Usually the larger frogs are recommended for beginners, like auratus, leucomelas, tinctorious so maybe check those out and see if they interest you. They come in many different colours. These are all terrestrial frogs, so would do better in a horizontal orientated vivarium. 5-10 gallons is the "recommended" size, but I dont like to see pairs of the larger frogs in anything less than a 20G aquarium, or an 18x18 exo.
The sharp area of your stump shouldn't cause any problems. Many of the broms used in vivs have spines and don't cause any problems.
As far as making a waterfall in the viv, there are TONS of threads on this on the forum. Use the search function and search "waterfall" or any combinations of that and you will find tons of info. Remember though, the waterfall is for your enjoyment, and is usually no use to the frogs. It also can take up a lot of useable space from the frogs.
There is generally no "cleaning" of the viv, since people use bugs to do this. Springtails and iso's/woodlice do a great job. Often you will have to trim back plants though otherwise it will become completely filled with plants, assuming they grow well. Usually this only needs to be done every 2-3 months, depending on plants.
Again this is just my thoughts on all of this, I would suggest using the search function as much as possible as 99% of the time the question you have has been asked time and time again by other members.