Extended QT might be possible, but I'd prefer not to have a new arrival go through pairing with an additional bug, itch, or bleed. QT first and before introducing to your healthy livestock.
I would prefer extended isolation of the pair after the new one has been QT'd, even if post QT pairing was in the existing tank.
In my experience, the newly added Clarkii will submit immediately and then the dominant specimen will remind junior frequently over the next few weeks. Could be vigorous, with torn fins, chasing junior. Make sure the tank is covered to avoid fishy jerky. After a few weeks they should be bonded well enough for a move to a new larger tank. Even then, I would prefer to divide a tank of 350gal size and keep them clustered together for a few weeks after acclimation. But that is just me being extremely cautious after many poor results over the years, most because I rushed things too fast.
Note: I had great pair conditioning results for breeding with a 300gal, not so great for pairing. Was too large and allowed a couple of stubborn specimens to stake out their own chunk of the reef and become females.
