Hi there! I also have a 40 gallon tank, so I'll just list here what I have for CuC because I'm happy with what they do.
1 big Turbo Snail (for the rocks and glass)
6 Trochus Snails (for rocks and glass)
5 Astrea Snails (for rocks and glass)
8 Nassarius Snails (sand sifter)
9 Hermit Crabs (just regular, gray legged ones, for getting beween the rocks)
1 Tuxuito Urchin (cool to watch, keeps algae under control)
2 Emerald Crabs (for algae)
1 Arrow Crab (awesome looking, great at getting decayed/dead matter out of cracks in the rocks)
1 Diamond Goby (excellent sand sifter)
1 Cleaner Shrimp (fish medic, picks at stuff on the rocks and sand, awesome personality)
2 Peppermint Shrimp (makes Aipasia disappear)!
The only issue I had is the Turbo and the Urchin tend to knock corals over, but thats nothing a little epoxy didn't fix. I figure I have all my bases covered plus even the CuC members are interesting and entertaining to watch. Heck, I only got an Arrow Crab and an Tuxuito Urchin because I think they are cool, but the fact that they clean the tank was a bonus lol.