I got one large one and two much smaller ones, and added them all at the same time. I've had them in my 200g display for only about two months or so, but I've seen no aggression at all among them, and they all hang out together in the same area of the tank. I also have a trio of bicolor blennies in with them, which makes for kind of a cool mix, since both species have similar color schemes (front half of fish one color, back half of fish distinctly different color). A successful experiment ... so far.