Fish stocking suggestion. Please?

[Jesterrace]

I'd avoid the Mystery Wrasse. They can be aggressive and even though sites claim they are reef safe, they will go after shrimp (including cleaner shrimp). 6 line wrasses are a big gamble. Some get ones that are decent long term and others get pecacidal maniacs as they mature. For a 40 breeder the number one wrasse on my list would be the Lubbock's Fairy Wrasse:

 

[OrionN]

If you have net cover, a few Flasher wrasse in a 40 gal is perfect.

 

[OrionN]

Flasher wrasses are peaceful and exceptionally beautiful. especially if you keep several males of different species. The will flashes at each other. Got to have a tight enclose screen top. They will jump out of any crack or opening.

 

[AdamNC]

Chalk Basslet, Bangaii Cardinal, Royal Gramma, Goby/Pistol Shrimp pair, any small Blenny.

I have a Bangaii and a Chalk Basslet and the Bangaii is the more aggressor.