I would go with a convict blenny. They are cryptic and hide a lot but they are very hardy and do not mess with inverts or corals. They look like an eel but they are very nice tempered and keep to themselves.
My convict blenny is about 5" long now and is getting bigger by the day. He never messes with the other fish. He has a very cool network of tunnels under the rocks and keeps himself in there a lot. He does occasionally come out.
This link will give you more info on them.