I was the one who posted the scopas/yellow addition. For me, both were identical sizes and the yellow had to be added to the DT with the Scopas. The Scopas was the aggressor 100% since he had been in my DT some time but it wasn't too bad until feeding time. I posted about 20 pics of various tangs (scopas, yellow, purple, and sailfins) all over the tank and within a week they didn't even pay attention to one another. In fact I've caught them at night in the same cave and schooling/feeding together during the day. I would never recommend putting the 2 together unless you absolutely HAVE to.....but it worked for me (and only a 93 gallon tank). The yellow is not planned to remain in this tank....but for the time being he had to stay there.