D. fantasticus are among the larger arboreal darts, but they're still pretty small compared to the P. terribilis, D. leucomelas, the tincs, etc. In my experience, all those I have utilize both the upper and lower regions of any terrarium. When you think about it "arboreal" isn't really a term one can apply to a terrarium situation, because there isn't a lot of difference between upper and lower regions. In the wild, there are many meters of difference between upper and lower regions.
While zoos and special shows mix for display purposes, it's safer and easier to keep track of each speci on an individual basis. There are differences in aggression, shyness, and voraciousness in eating that are simply too difficult to keep track of and control in a mixed tank. Some of the larger frogs like the tinctorius, leucs, galacs and P. terribilis are virtually eating machines. It's not a question of cross breeding. The arboreal thumbnails have such different breeding habits from the others that cross breeding would be highly unlikely.
As someone mentioned below, I wouldn't dream of putting tiny $150.00 (very territorial) D. pumilios in with big fat $35.00 P. terribilis that wander around anywhere they please. Some darts are groupies among their own species, while others get along well only in mated pairs. My advice would be to try a couple of different species in separate vivariums and get some experience with them before even thinking of mixing them. You probably won't want to.
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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