Really it would be up to you. Personally, I like to set up a tank with a group of free swimmers, rock dwellers, rock perchers and bottom dwellers, that way you have the various areas of the tank populated. This being a FOWLR
, you don't need to be concerned with fish being reef safe, so that really opens up your options. My personal list for a 40B would be the following:
Basslet--rock dweller/free swimmer
Randall's Goby--bottom dweller
Check out this site.
You can filter your search based on your needs. It's a great resource when setting up a stock list.