The sizes you are describing in my experiencie are quite small, the smallest adult blue auratus I had was a bit more than three cm, a male, but normally not less than 35-38, adult females more than 40 mm. For example, all my offspring from last season that is now around 6-7 months, they're all around or easilly more than 35 mm. Are you sure about their age?
I personally think that sexing them, even if they are adult frogs, it's quite hard, sometimes not, but I think it's something really hard to assure.
In my experiencie, I don't know about other ones!
I personally think, talking about my experience with this frogs, that this viv is too small for such a large group, I kept for more than three years a breeding group in a tank a bit bigger than yours, with a lot more usefull surface and hiding spots, and big fights where in there (originall a 2.4 group), it became much better when they became 2.2.
But as they told you, you have to take an eye on them, if nothing happens, no problem, this was just my experience with this breeding group.
I also find that the keeping temperatures are a bit low, with this values, my frogs would be much more letargic. I always kept them around 24-25 degrees during the day time, same with offspring, and even if they were a bit shy, specially at the beggining, they where really active frogs.
I think as well that some leaf litter would do really fine in there.
Just my thoughts
greetings!