If you plan on taking the reef path for your tank, the Black Diadema Sea Urchin makes the best choice. The reason being they are not cumbersome so you won't have to worry about it knocking over your corals and rock, that is associated more with the other more destructive urchins.
With inverts that make up a cleaning crew, more importantly, such as Sea Cucumbers, Urchins, snails, Mandarins and Dragonets/Scooter Blennies, patience s a virtue. A mature aquarium will provide the best chance for success with these animals. Algae, pods, plankton and detritus will have a chance to develop which will provide the necessary foods and nutrients for these unique animals.
I would start with Emerald crabs, hermits, Nassirius snails, ornamental shrimp and sea stars( Serpent and Brittle) and then from there you can expand.
Good luck!