With your semiaggressive fish I would not add the following suggestions: Helfrichi Firefish (or any firefish for that matter), Flasher Wrasse, Assessors, Jawfish, and even a fairy wrasse may be a problem. Your Clarkii and blennies are probably going to be a huge issue adding another fish that docile. I would probably suggest going with a larger dottyback, kole tang, or a larger anthias species such as Squamipinnis.
I didn't even think about that. My Clarkii roughed up my blue midnight angel the first day I added it, and will still chase it away from his anemones. A really passive fish like a firefish or jawfish may not do so well...
I might just stick with a purple tang after all...