To be honest you still risk them drowning when they're very young - I could see a situation where they might even get stuck under the large secondary platform.
If you're going to go with musks or muds then the best way to go is either to buy them as hatchlings and raise them in a seperate container for the next, say, three years OR re-home a couple of adults or sub adults. Even small species like oderatus get too big for some people who buy them when they're adorable and the size of a 20p peice. I've seen a few around at 3-4 inches recently.
However, the idea of a tank with more than one level is quite an intriguing one for adult musks. I was going to say that (as previously noted by Alex) 12" water depth is about the max you'd want for adult musks, (I'd even say that 8-10 may be a better bet), but a 15" tank with a, say 6 or 8" level and a basking station that should be great and will increase the possibility of fish.
As I have previously noted Musks aren't great baskers, so your medium dock should be fine. However, remember that to keep terrapins you shouldn't have any kind of lid on the tank, and you need some way of fitting a UV and a heat lamp.
In terms of fish species, I have kept gold barbs very successfully with muds and musks, as well as white clouds and zebra danios. It may be a bit of a trial and error thing - the chances are that if they can catch them they will so don't take too many risks with slower species.