So much for not adding new fish, eh?
Your Powder Blue picking on a fish for a day is NOTHING! I think your Powder Blue is an ameteur bulley in comparison to my Achilles.
The key to adding new fish with these guys is making them forget their territory. If the fish is different enough fron the tang, like the wrasse, you can let it go through it's initiation day and things should be fine after that. Although, if you quarantine the fish and fatten it up, it will be better equipped to stand up to the abuse. Adding all the new fish at once will divide the aggression and give each fish a little more peace. It will also reduce the chances of any of the new fish fighting with eachother.
If you happen to get a fish that the tang will be displeased with indefinitely, there are a few things you can try. If it is a fish that is known to be difficult to get eating, I would highly recommend quarantining it just to get it eating well before adding it. Otherwise the abuse from the Powder Blue will make it less likely to eat. I've gone through this several times with Copperbands. If the fish is small, chances are it will have an easy time fleeing and hiding from the Powder Blue. Just make sure you add the fish at night to give it a head start finding it's way around the tank. I added a Scopas Tang to my tank and because he is a lot smaller than the Achilles, he has no problem getting away from him, and gradually the aggression is subsiding between the two. If the fish is bigger, try using egg crate to divide the tank in half and leave it this way for a month or two. This allows the new fish to acclimate to the new surroundings in peace, helps the Powder Blue forget about that half of the tank being his, and will get the Powder Blue accustomed to seeing the new fish without being able to attack it. It can be a pain to set up the egg crate, but I think it works great. I've done it several times and I think it's the only reason why I was able to add a second Acanthurid to my tank.
If you consider getting an pygmy angel, be prepared for your sps polyps to always be retracted.
For some smaller fish, how about an orchid dottyback, a royal gramma, a clownfish, or a midas blenny? They are all small fish that are colorful and/or fun to watch. Things to consider - the dottyback is aggressive, but it's small, the gramma is timid, and the blenny is a jumper.
If you can find a wrasse that doesn't burrow, that would be a nice option too, but you might have aggression problems with your 6 line.
Or how about an Anthias? They need to be fed a lot and tend not to do as well in brighter lighting, but they're very colorful.