I'm gonna be honest and say you should add a smaller boa and grow with it. They are typically super docile and not as jumpy and are voracious feeders. If you get one that has a bit of handling experience and isn't a true hatchling, there's very little to worry about. They are also very hardy. As far as size goes, you obviously don't mind since you already have a STP!
Carpets can sometimes be a handful and sometimes move a lot. My boy really doesn't care for being handled and moves so much so it makes my girlfriend very nervous. I've heard some of quick-to-defense carpets, but mine is just vocal and prefers running from me instead.
You already know what you're getting into with a STP and BP, so I won't comment other than I started with BPs and will not be buying anymore solely based on the lack of how little they 'do'.
My $0.02.![]()