o hai there...
I have a 40G as well, and keep the following:
-1 Mystery Wrasse
-1 Bicolor Blenny
-1 Yellow Watchman Goby
-1 Cherub Pygmy Angel
I love my setup. They all get along great and each have amazing personality.
From your list, I would eliminate/select... (Personally)
-a goby shrimp pair = Would work Great. Yasha Hashe's or high-bandes are my personal favorites.
-royal gramma =
-Mandarin goby = Eliminate
, for above reason.
-clown pair = Obviously great.
-firefish = Great.
-yellow headed jawfish = Needs a DEEP sandbed in order to thrive, unless you make a custom burrow.
-yellow headed sleeper goby = Hard to keep. Eliminate.
-trio of banggaii cardinals = You will most likely get fry eventually.
-cintron goby = Size issue (Small). Eliminate.
-green clown goby = Stays too small for a 40G in my opinion. Eliminate
I would get the clown pair, goby shrimp pair, a firefish, and the trio of Banggaiis personally.