Wayne should be along any time now, if his macro is working properly
I agree. A 6ft tank with 300 pounds of rock isn't any more suitable than a 36 inch tank.
Of the two you've mentioned, the YT would, IMO, be more suitable. The PBs tend to like really blasting around the tank.
Another point to consider is that while yes, Yellows can get to be the size of dinner plates, I've never actually had one grow much beyond the size I purchased it. I feed a lot.
So if you have a smaller tank with tons of swimming room, add a YT. Keep the water clean and feed it well to ensure good health. While there are tons of rules about this online, Tangs don't read a lot online and don't know the rules.
Get a smaller to mid size, make him the primary large fish for the tank, keep him healthy and you're doing fine.