I have both in my system.
Yellow was well established then I put the Purple in.
Was worried because the Purple was big (maybe 4 inches), bigger than the Yellow.
The Purple came from another reefer's tank where it totally dominated.
That's why he was selling it. After I paid him, he starts telling me it even ate a chromis he introduced. I was like - WHAT THE HELL HAVE I DONE HERE !!!
Dropped it in the tank and the Sohal quickly sorted it out. Soon the Purple and the Yellow became big buddies, maybe plotting to overthrow the Sohal - LOL!
Its a 300G system, so enough room for the aggression to be dispersed.
But I must admit I was worried over the outcome.
If I had to do it again, I'd put similar size fish in at the same time.