I don't know if I would stick a blenny in a 20g, you are just on the small side for one of them. However, Carey gives great advice, so, if she recommends one, you are probably fine.
Don't just go with all hermits for your CUC, you need something that can clean your glass for you. I found that astrea snails do the best job for this. Stay away from turbo snails, with a small tank, your chunks of rock won't be so big making it easy for them to knock stuff down. They do get pretty big for a snail. Split your CUC with about 3 hermits, 4 astrea snails, 6-8 nassarius snails (they keep sand stirred up by burrowing), and if you want a shrimp, you could get away with a cleaner shrimp, or a pair of peppermint shrimp.
Also, remember to keep anything such as rocks on the bottom glass, and then pour your sand around it. This helps keep rocks from sliding due to things burrowing under the sand.