here are a couple of suggestions;
a longnose hawkfish;
they get about 5" max, are not constant swimmers and are chock full of personality. They are on the low end of medium aggression but will go for really small fish/crustaceans if they get hungry.
Polka Dot or Pajama Cardinals;
they get about 4"-5", are really mellow and can be kept in small groups if desired.
Same with the Bangaii Cardinal also in the pic, although Bangaii's tend to be a little more "assertive" without being a menace.
they also bark at the other fish.
(if you have Bangaii's and hear strange "thumping" or "barking grunts" coming from your tank, it is probably the cardinal expressing his opinion to another fish...LOL)
A Lemon peel or Yellow angel would be good as their bright color will stand out from across the room. I have a yellow coris wrasse for just that reason.
Same could be said for a Flame Angel as well.
Then there is a slew of interesting Dottybacks and Basslets that would be the right size for your tank. I saw a Royal Gramma in a store a few weeks ago that was almost 6", never saw one that big in captivity before. They wanted $50 for it.
You just need to research them because some are mellow and some are sociopaths.
I would steer clear of any damsels except clowns because most invariably because bullies.
there are a few Anthias that could also probably work, just make note of their max size as some get pretty big.
I agree, finding a good mix in that size can be a challenge.
In my 50 I finally settled on;
yellow coris wrasse
6 line wrasse (he may get evicted)
green mandarin (I got lucky, this one eats like a pig)
and a Fiji damsel that was the first fish I got in this system.
they are all relatively the same size and aggression level, there are occasional "get outta my yard" type of squabbles, but no real nastiness has developed between anyone.
It is funny to hear the cardinal go around "barking" at the other fish as he struts his stuff.