Well, if it's not high enough for red eyes, then it's not high enough for either milk frogs or mossy frogs. They're all tree-dwelling frogs as you know. Mossy are no doubt the largest of these frogs, so if you're short on space I would advice against mossys. They also need a real water part (preferably with moving water) which takes up even more space.

Lastly, I have to say that I'm not very fund of the idea "I have a spare tank, what can I put in it". Decide what pet you really want, then get an appropriate tank for it, not the other way around.