Your tank sounds amazing Kat! I would love to see it when its done. You have a very big tank which is good because the more room the better for tree frogs.
Most tree frogs aren't the best swimmers but if you want to have 6" of water, make sure you have many ways for them to get out. I have a half land half water vivarium for my whites tree frogs and i filled the water part with some pebbles and when they get older, i will remove the pebbles so they have more room.