I'd skip the urchin. I've got urchins in most of my tanks, most of them eat coraline, but they also end up spreading it around the tank because some of the small bits they miss get into the water colum, so in large tanks the coraline growth easily outpaces what they can eat. In a nano unless your rock is absolutely covered in coraline and you're seeing good growth on the glass, it may not be growing fast enough to keep up with the urchin, and it will simply strip what you do have then starve to death.
B