Someone else might chime in too, but given your current setup I think you are light on the substrate dwellers (except for the mandarin). An interesting addition would be a shrimp goby (like your watchman idea) plus a partner shrimp. I have two of these pairs (different species of gobies) in a 55gal and they are sure fun to watch. No conflicts between the shrimp (which are both Randall's pistols) so far.
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