I LOVE orchid dotty backs, great fish!!!! I don't know if your royal gramma would be peeved about the introduction of another purple fish though. I have never had a royal gramma and I read contradictory reports about how they accept newcomers.
As for anthias, you will have to do a lot more research about what you could keep as you are limited with your tank size, if you get a chance pick up the 1st volume of "Reef Fishes" by Scott Michael. It will answer alot of your anthias questions.
I would keep it to 6 small fish total.
Orchid dottybacks are GREAT fish, one of the best. However you should rearrange your rock if you introduce one. This will minmize potential problems with the royal gramma. You might even consider adding a pair.
Other options, endless really - but here are a few of my favorites for a reef tank that size.
Pygmy angelfish - C. argi
Flameback angel - C. acanthops (chose either this or C. argi, not both)
Sixline wrasse
Ocellaris clown (no, don't call them false percs. Percs are false ocellaris!)
Chalk bass - S. tortugarum
Marine Betta or Comet - C. altivelis
To add another clown...
Use a small clear container with holes drilled in it. One of those cheapie specimen containers with the colored top will do.
Set the container with the fish ( that you have quarantined first of course) in the tank and let your existing clown get used to it for a few days. Buy a SMALL clown, not one of equal size.
I just established a maroon pair this way, and they are much harder than ocellaris.