What about a Clown Goby? They are peaceful and nice-looking.
I like Sixlines too, but mine would sometimes nip at a fishes fins. My Sixline also hated my Royal Gramma, so I had to take the Gramma back. The funny is that I see Sixlines and Grammas together all the time and they don't fight. Maybe I had a bad fish, lol. Or maybe it was because I added the Sixline before the Gramma.
Also, my Sixline only nipped at fish that were added the same time, or after him, so that might be a factor also.