Kole tang or 2 or 3 cardinals. And a goby too - what kind you ask?
First, I gotta ask how you six-line does with the bicolor goby since one hangs our and perches on the rocks while the other constantly cruises them. I've heard of the six-line claiming that territiory and not liking someone else perching or cruising.
The clown goby is another "percher". With or without a pistol shrimp, you could do a shrimp goby who'll hang out at the very bottom between sand and rock. Plenty of them to choose from. With that six-line already established, I'd be cautious about another that will perch.
You could look at the fang blennies. The canary is my favorite. He'll give you that bright yellow you prolly wanted in a tang, but couldn't because the 75g isn't big enuf. He'll constantly swim out front. He even sleeps on the front or side glass. And don't worry about the fang part. He's only dangerous to other fish trying to eat him or the uninformed aquarist who might try to hold the net he's in while transferred. There's another fang blenny that has pretty shades of pale blue. Both small.