I have a fourline green wrasse which seems to be getting more aggressive by the day. I put a YWG in the tank with it and it was a non-stop battle for 4 hours that the goby lost. Probably won't affect you with a BB tank though.
I had been considering a chalk bass and/or a purple firefish. Not sure if they are compatible with a 4/6 line, but they are interesting fish and would probably look great in your tank.
Other interesting small/medium fish:
spotted hawk (great fish, will watch you as much as you watch it)
marine betta (likes to hide, but one of my favorites)
clown goby
dottyback
pipefish
blennys
royal gramma