This is for my Chinchilla. It is a Super Pet XL on the bottom. I then bought another brand's 3 level cage and removed the top hatch on the Super Pet and wired the top of the 3 level to it.
Actually it is very stable. Thanks for the concern because you never know! Chins weigh next to nothing and Buddy is more like a dustball flying around in it than a ball!
I did the same sort of thing for my hamsters. It's great being able to stack different cage tops onto each other, and then remove the top doors for entry to the next level. And it looks so nice. You did a great job using such a large cage on the base. What a nice home for your chin! Kudos!!!