A 15" high enclosure could definitely work for a carpet if you wanted to add another. I would almost never advise against a carpet. It doesn't matter to them if they're 6" or 6' off the ground. My carpets are in 12-18" high enclosures and they're just fine. That being said, I think a bull snake or boa would be a nice addition as well. Since you like the Aussie snakes you could also go with an Olive Python. They're amazing snakes to keep but you may need a 6' enclosure down the road.