I have had great success with bicolors in groups and I understand that terribilis do just as well. I've also kept leucomelas, but they seem more shy than the bicolors, which are very bold and in the open all day. I also have a nice group of Ameerga bassleri and they do very well, with each one perching in its own spot observing a subtle pecking order. If you would consider thumbnails, I used to keep variabilis in a group of up to seven in a 20-gallon viv and they thrived. You could probably put at least 12 to 15 in a viv that size. I hope this helps. Also, some say that auratus do well in groups, but I found that over time certain timid frogs would decline and disappear leaving a pair or trio of survivors.

Groups that start out together do best and you can likely count on a few losses, so if you want a group of say 5-6 of the larger species, I'd start with 7-8 juveniles. The ameerga and variabilis will make use of the drift wood, whereas the other species I mentioned will mostly stay on the ground. You might want to consider that when you decide on a species. If you get primarily ground dwellers, make the floor a little more complex that will allow individuals to rest out of direct sight of the dominant frogs. Sometimes strategically placed plants or small changes in bottom elevation help reduce the stress that some groups experience.

BTW - really like your viv design.