-fwiw, I don't think you need an established tank. I put two in my Red Sea 650 after only a couple months, but it was totally finished cycling.
-don't underestimate the mess they can make. I love my two, they are endlessly fun to watch. They also bury EVERYTHING on the sand. Had to move all my acans onto rocks. They make sand piles 4-6 inches high all over the place. They make bombing runs up the rocks vertically, strewing sand all over them. In hindsight I would only get one, since they mess up the tank so much.
-they absolutely clouded up the water for the first month. Now it couldn't be clearer.
-fwiw, mine eat everything except nori sheets. Heck, they probably eat that too. Pellets, mysis, herbivore mix, they are total chowhounds.
All that being said, imo they are just a terrific fish!
Ps mine are orange spots and I have Caribbean reef sand