If you want the display to remain a reef I would not put Caulerpa of any species in. Halimeda, Udotea (mermaid's fan) and Penicillus (shaving brush) are good options. The fleshy reds: Halymenia, Gracilaria, Botryocladia, Palmaria and a few others are good candidates for darker spaces in your reef as well. I believe a few people have started putting Codium in the display as well, but I'm not sure how that is working out in all situations.
>Sarah