[cypher]

If you want to use different kinds of substrate that's fine as long as they're all safe, I wouldn't section areas off though as your ham needs to be able to burrow, the base of the cage needs to be filled up to the top, that will give a reasonable burrowing & nesting depth.
You can put the wheel on a shelf or on top of a large wooden flat topped house to keep it stable & free of the substrate, there's plenty of height with the plaza so that shouldn't be a problem.
Have fun setting it up!