i see Scott covered mostly everything pretty darned well. but i might be able to add a bit for you. as far as the snails go, i'm not sure about parasites, but if i were going to give snails a test with my white's, it would be malaysian trumpet snails. they stay under the substrate most of the day and only venture vertically at night for air. in a pond, i would suspect they would just crest the water then head back down. i don't believe they will venture onto land.

unfortunately, there are no small, true lilypads, but you can use amazon frogbit. it resembles it very closely in miniature vivaria.

as far as cleaning the bark. depends on whether or not you use a misting system and or have plants growing on it as to how often you clean it and how.

oh, and the best thing you can do for your frogs is research their natural habitat, ya know, where the guy above me is from. ^^ btw Scott, is it true your toilets flush counter clockwise??LOL

good luck on your build and keep us posted