I have always found a Royal Grama to be an excellent choice.
Six Line Wrasses are usually fine. I did, however, find that mine suddenly took a dislike to my Mandarin, and beat him up badly. I have two tanks, so I was able to remove him.
I have two Purple Firefish, and have found them to be very mellow. Strangely enough my Bicolor Blenny took a dislike to one of them and ripped him up badly, so he too got moved to the other tank.
Now both my tanks are mellow.
