Think you are going to find that each pair of clarkiis are going to have their own personalities. Mine are pretty calm, the only time they were mean was back when I had a 3 stripe damsel in my tank, they didn't care for him at all. I am thinking it was the stripes that did it.
Recently I moved my flame angel in to that tank. The only thing that they did to it was sometimes get in its way -- if the flame was trying to get into a cave they would block the entrance. I swear I could almost read their minds, they had a look about them that was saying that -- "we really don't want to do this, but since we are suppose to be mean we'll make a token effort at being mean."
Though I believe I have a mellow pair, when I moved the tank they were spawning later that week, since that was their schedule. They will continue to spawn even when I have my hand in the tank doing a water change.