Livestock in refugium?

[reef bound]

If I looked real hard I could probably find something to tell me this, but I'll ask you guys instead. Is there any particular critters that a person should or could have in their fuge to make it better or more "productive"? We've got the various algae types and some snails and baby brittle stars and a couple of (I think) rock urchins, but is there something bigger we should have to make it better?
Thanks for info.