Personally, I'd say your tank is overloaded already from a space standpoint, so I couldn't really recommend another fish, even if it was compatible. At least you have an extra 10g of water to help you out with water quality.
One thing with the Banggai is that while it's "peaceful" I've noticed it can be fairly territorial. So in a 10g tank, that could become a problem with any tankmate. I've got a Banggai with a single O.Clown, three chromis, and a little clown goby. The Banggai seems to go through phases now and then where he'll chase the chromis all over. He'll make a cloud clicking sound as he snaps his jaws together. No nipping, just showing them who is boss. The chromis are good at ignoring him. I'm pretty sure there'd be mayhem in the tank if he was in a 10g.