Hi, welcome to the forum! I think I can help with your questions. And don't worry about rambling...I make massive rambling posts all the time. In fact this will probably be one! First off, White's Treefrogs(aka Dumpy Treefrogs) make a great first time amphibian. I doubt they will drown in your water feature. I've even heard they love swimming. The general rule for how many frogs is 10 gallons of space per frog, your tank is 100 gallons, roughly, so you could have as many as 10 frogs. A little extra space is always appreciated however. I'd go with 5-7 frogs myself, although you can certainly have less if you like. Dont hesitate to ask if you have any more questions. Depending on stores in your area, there may be other options. Grey treefrogs are cool, and I keep Pacific Treefrogs, a species native to my area. Good luck picking a frog!