There are lots of great fish you can get for your 40b. You can look through the fish list of Live Aquaria in their nano section here
http://www.liveaquaria.com/product/aquarium-fish-supplies.cfm?c=15+2124
And there are many other small fish in the main section here:
http://www.liveaquaria.com/product/aquarium-fish-supplies.cfm?c=15
Put together a list of fish you want and post it here. We'll let you know what will work and what won't as well as have a few suggestions.
I've got a 40b and have the following fish in it
Cherub Angel
Midas Blenny
McCosker's wrasse
Matted File Fish
Royal Gramma
Pair Mocha Ocellaris Clowns
Yellow Candy Hogfish
Scooter Blenny (just got him today :dance: )
Also let us know how your tank is setup. How long , sump and equptment.