Use the inside holes for dust extraction unless you're sanding something that will produce big particles/chips like removing popcorn. Turn the CT all the way up and use the dial on the Planex to regulate suction to find that sweet spot where it holds to the wall to some extent but also floats, probably around 3-4.
Start the Planex while it's ON the wall rather than turning it on and bringing it against the wall, that will minimize the chance of nicking it with the edge of the abrasive/pad. Start slow, there's a learning curve, remember you can dial the speed down on the Planex easily. Also, start with a high grit until you get the hang of it. It can really motor through some drywall compound if you want it to.
If you do a lot of overhead work, consider the harness as an option, it's worth it.
The mount for the CT is a nice-to-have. It holds some abrasive sheets and the Planex when it's not in use. The neat thing is the Planex will go on it pad down or pad up. Pad up is nice for removing and applying a new sheet of abrasive.
I've only spent about 45 minutes using it and that's what I can share to get you up and going quickly.
If you have other specific questions, feel free to ask.
Shane