I don't think a 55 is too small for a snowflake, a couple of years ago I had a 55 gallon with a snowflake and he did fine, grew up to be pretty big, they don't really move around all that much as long as they have enough rock to hide in, I would go with some damsels that are big enough so that the eel won't eat them, and any other small species of fish should be ok, as long as you keep the overall number of fish in your tank around 3-5 at the most.