Been using the Exped Megamat Duo 10 mattress in our ARB Simpson III for about two years. No problem and much more comfortable than the stock mattress. We just deflate it slightly before packing the tent.
We tend to use blankets in summer conditions, but later in the season when temps get below freezing we prefer high quality sleeping bags. Close fitting sleeping bags are more effective at trapping heat and they tend to minimize cold drafts when you're shifting around. That's been our experience.
Just don't drill too large a hole and you should be fine. We added three sets of holes (adequately spaced) to our ladder to allow for different terrain. Almost five years and no problem.
^ This. You have to drill some extra holes if your ladder in not fully extended. We've done the same thing because we've needed to add a short ladder extension.
I bought an ARB Simpson III RTT and had to cut the cover at the end of a trip when it got caught in the zipper. Called them to buy a new cover and not only did they send me a new cover for free, but they also sent the new pole assembly they were now using in the tents. Great company and great...