Very nice frogs. I honestly don't know why I haven't picked up some cobalts. They are my second favorite tinctorius morph - azureus being my favorite. I'm swapping some of my green & black auratus tads for some powder blue tinc tads this Saturday at the Daytona show, but I like the cobalts better.
I'd build a 29-gallon viv for the cobalts and save the vertical tank for your first thumbnails (they're awesome).
Hopefully you'll end up with a pair. When they mature, the odd one out might have to be removed. Some folks have luck with tinc trios that are raised together, especially when there is only one female, but most of the time you can only keep one pair unless you set up something really big - 75-gal and up. The females will fight and eat one another's eggs.
Watch them carefully and make sure that one doesn't start losing weight compared to the others. You might not see obvious signs of aggression, but as they mature a dominant frog can subtly intimidate its take mates, keeping them in a constant state of stress and out-competing them for food.
Good luck and thanks for sharing.
Jim