Either fantasticus or reticulatus would do well in this if you add a couple of small Neoregalia type bromeliads to the background and remove the Spanish Moss. Just use grow-lite type lights for the plants. Darts don't require UV and a UV light source may be detrimental to them. I don't know about mantellas, however. I just acquired a new reference on amphibian physiology, but haven't delved into the UV aspects yet. I'll report back when I can cite some studies and and referrences. Some frogs do require a UV light source for calcium utilization, but darts are kept without it quite successfully.
Although I've never had trouble with the Spanish moss personally, I removed it from most of my tanks after reading a couple of reports of frogs becoming entangled in it with subsequent injuries. It's pretty wiry and can wrap around legs, etc.
In my experience, the other frogs you mention as alternatives either require more ground space (although all will do some climbing) or in the case of pumilios, need more bromeliads that hold water, as these are essential to their comfort. Pumilios, at least my Bastimentos types also utilize the ground area more than any other of my thumbnails, and are best kept in pairs, especially in such a small tank, because they are very territorial. I have each pair in a 30 gallon tall and they use every bit of it.
That's a very attractive terrarium, though. You should have no trouble adapting it. If you already have a good gravel drainage area, you can sump this out regularly with a length of aquarium airline tubing to keep the soil from becoming saturated and stagnant. However, false bottoms are best for waterfalls and in-tank water features. These are best incorportated into the original design.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus