Hi!
Would love to see the set up when you're done - we are currently in the process of building one as well!
I'm not really positive as to what you're trying to do but if I understand correctly, you have tree frogs and you might get a crestie and place them both in the same tank? My advice would be to not do it. Cresties tend to think every little moving thing is food - from fingers to even hair. You're frogs would probably not last very long! Also there is the issue of poison. I don't know anything about the kind of frogs you have but I know that some frogs can have poisons on them or in them. I would definantly not reccomend putting the little guys together. I hope this was helpful!
Cha