I think clown loaches get really big, like 10" or so...but it also takes a while for them to get to any appriciable size. They also like hidding spaces. I would recommend a smaller loach, but im not an expert in loaches, so I cant recommend any.
I happen to be a natural aquarist, but I think youve got a neat idea. The unnatural aquariums can look really cool if you stick to a theme and do them right. I saw a 55gal tank at a college party one time that housed tinfoil barbs (I think... it was a while ago, and for some strange reason my mind doest remember it all that well) It had black gravel and sunken highligher filled absolute bottles. It didnt look natural at all, but it was a really beautiful tank!
I think what your going to want to do is use very fine gravel. In fact, I would use sand for the moat. One issue will be keeping the moat gravel in place so it doesnt drift over the land gravel.
By the way, Im not sure where, but I have seen some amazing pictures where aquarist take outdoor landscapes (think beach, or a waterfall) and recreate them underwater. If you can find them, they might help you figure out how to do yours since that seems to be your plan.