I just bought a pair a couple weeks ago. They were already a pair though, not sure if you can put two individuals together.
The tank is an 11 gallon, with anemone shrimp, pygmy gobies, and mushrooms/zoos. (larger shrimp/fish might cause the Harlequin shrimp to hide a lot). Mine are always in view, but with a hiding spot close by.
I fed mine a sand sifting starfish, but I've seen them eating chocolate chip starfish at the local stores. I'm trying to get some asterina starfish breeding in my other tanks, but not much luck yet.
Hope this helps.