I always had a single blue tuxedo urchin in my system, all the way from my 30 gallon setups to my current 115 gallon build. I never had issues with them not getting enough to eat.
I also never used any type of media or chemicals to lower my phosphate levels in my systems, just macro algae in my refugium. Plenty of algae and other things were always available for him to eat.
From my experience, coralline grows quick enough in a mature system that it was never an issue, no matter how much coralline they might have consumed.