I like the butterfly idea for your tank. They are peaceful, and will not put a huge bioload on your aquarium. Plus, your tank is now established enough to try a butterfly. The only problem with doing a butterfly is that you will have to find one that is eating aggressively at the LFS for it to have a chance in your tank with all of your other fish. You do not want a shy, passive one.
Pearlscale butterflies are one of the better, hardier chaetodon butterfly species. If you find a healthy specimen that eats well in the LFS' tank, this would be a good choice.
Another great, hardy butterfly species is the racoon butterfly. These usually fare very well and only get about 5 inches in aquariums.
The Merten's butterfly (Chaetodon mertensii) is one of my favorite butterflies. They are very hardy and usually eat and adapt very well also.