I'm quite a big fan of urchins - they eat a lot of algae, and generally are quite careful moving around. The Diadema pincushions do get terribly big, though, and just have too much diamter for a standard 55.
I really like Mespilia tuxedo urchins a lot, they get thro a lot of algae, and aren't too fussy about what type either. I've never seen them on my desirable inverts. they don't get too big, and I'd recommend them.
I also like Tripneustes pincushions, and I also have a rock boring urchin - these are good as they will remove hairlage holdfasts, and I'm not too worried about losing a bit of coralline here and there.
I know people are a bit shy of urchins, but I prefer them to hordes of snails, and pesky hermits. Hermits are far less reefsafe i.m.o. The only hassles with the small urchins are acclimating them right, and aking very sure you DON'T GET SPIKED.