First of all, talking about a tank in gallon really is... suboptimal, if you ask me. Dimensions is the way to go, since different species might have different requierments (as I'm sure you know). BUT, since you only talked in gallons, I might as well share my view on this. I wouldn't put any tree-dwelling frog in a 10 gallon for anything othrr than a temporary quarentine-tank. Even a 20 gallon seems too small in my eyes, especially if you want mossys, since you'll need a large pool area with enough water for them to swim, and with aquatic hiding places. Lastly, mossy are quite much larger than red eyes, increasing their need for space.
Note: I'm assuming that by mossys you are talking about Theloderma corticale . If you are talking about some other theloderma species, I can not help you much.
My "knowledge" are only from articles, books and other texts I've read, which states that they (red eyes) are poor swimmers. However, I've not tried to throw them in a deep pool of water, so I can't say that I've seen them being poor swimmers. Do you have a reference to a study (or similar) that say anything about this? Would really be interesting to read, since this seems to be a topic with a lot of different opinions.