That's a really nice list - that's an impressive community. The assortment of tangs is particularly terrific.
Looking at the list, I see the powder blue, the hippo, and the copperband bfly as the most delicate fish in there - and the Achilles? I never was able to pick up an Achilles Tang. They're a favorite of mine, too.
The copperband is just hard to keep due to eating requirements. If you have that going well, Kudos to you, and he's probably not going to get stressed too much.
The Hippo and Powder Blue are particularly prone to ich, and it's my personal opinion that stress is the primary culprit with those two species. IF it was me, I'd want to pick a docile reef-safe wrasse to keep stress levels down.
You do already have a royal gramma, which, if it's a small tank, may beat up any similar-shaped fish that you add, including a small wrasse.
SO.. again, if it was ME, I'd want a community wrasse as to not upset the hippo and the powder blue (and the Achilles, I think, they may be delicate, I never had one), but also something that could stand up for itself to the Royal Gramma.
I wouldn't get any fairy wrasse, they're too different than what I've had success with. Maybe a 4-(Pseudocheilinus tetrataenia)or 6- (Pseudocheilinus hexataenia)or 12-line (Macropharyngodon meleagris)wrasse, or a Leopard Wrasse (Macropharyngodon meleagris) or a Melanurus Wrasse (Halichoeres melanurus) ?
I know it is very common, and he may not fit into your extraordinary collection, but the 6-line may be the easiest candidate to try. Cheap, bulletproof, and particularly non-aggressive. I currently have one that I inherited and he just cruises and causes no trouble at all.
Fortunately, the Yellow Wrasse I've been promoting (Halichoeres chrysus) is also pretty tough, docile, and cheap. he's currently on LiveAquaria for $15. I found the ones I've had to be pretty reef-safe, as well.
Hope this helps -
G.