There are quite a lot of different fishes you can add to a 55g, just depends on preference. Link to compatibility chart at the end of the post.
CUC should be added before you add your fish and only after your tank is done cycling. If its still cycling, just wait. I personally don't really like snails, so the only kind I have are nassarius snails.
There are quite a few shrimps like fire shrimp, coral banded shrimp, cleaner shrimp, peppermint shrimp, etc. If you decide on the coral banded shrimp, its aggressive to other shrimps so keep that in mind. The other shrimps should be Ok with each other.
You can also add hermit crabs as part of your CUC.