The yellow tang does indeed look to be on the skinny side - you can see his ribs. I'm not the tang police, but I'd strongly suggest getting him out of there...he's definitely got to the point where he needs a few more feet of up front real estate to prowl.
I'm going to disagree on the hippo, though. To me that looks like one of those quarter-sized hippos that my LFS gets in from time to time. From what I've read and seen at the store, these guys have a lousy survival rate - they're essentially babies and they're fairly weak. One of my LFS buddies (the guy I trust) said he'd tried one of these in his 220 and it died from being chased by the bigger fish.... basically it wasn't given a safe haven to eat and grow. A smaller tank would be like a nursery as long as it had peaceful fish in it. So yeah - if I had to I might keep a tiny hippo in a 20L until he grew to about 2.5 to 3 inches. But then I'd MOVE HIM TO A 6 FOOT TANK!!
Now, other than the "tang issue" that's been done - the tank itself looks very nice! That big torch of yours looks beautiful.
just get the yellow tang out please.