You can dunk them in a 5% bleach solution and then rinse them off and allow to dry. Pothos is actually epiphytic and can survive quite well in a humid environment without any soil. It can also survive indefinitely in a glass or bowl of water (this is also an easy way to propagate it via cuttings). Or you can just put it in a little pot with whatever substrate you plan to use for your vivarium.
If you aren't planning to separate the frogs then I'd just use the 12x12x18 they're coming with, but would replace the substrate. I wouldn't do anything fancy to it, just pothos and a few branches for climbing, and cover up the back and sides for added security. And get working on that 18x18x24

.