I'd hold off on algae eaters for now, your tank actually looks a little too clean for me to want to add algae eaters. Adding a conch plus a few nassarius would be a good idea to stir your sand a little, which should help with your sandbed cyano. Maybe a couple of cerith snails if you want to increase the supply of algae eaters a little.
I think a tuxedo urchin would be a fantastic addition eventually, but I'd hold off for now; I don't think your tank produces enough to keep it fed at the moment. I personally would recommend against a sand-sifting goby for your size of tank; if you do get one, get a Rainford's rather than one of the larger 'sleeper' gobies.