I haven't had a problem with them in mixed flocks with other heavy weight breeds that had average temperaments.
The time I had a problem was when I raised some light weight breeds that were more aggressive and high strung, along with a mixed group of heavy weight chicks. The more aggressive breeds were aggressive not only to the Faverolles, but to all the less aggressive breeds, like Plymouth Rocks, Sussex, ameraucanas, etc. They had a lot more space than most people give them, too. I had to have my friends come get them before they were ready to lay, as they were just too aggressive to the others.
So, I would look at the breeds charts and try not to mix much more aggressive breeds with less aggressive breeds. If you're only getting the average heavy weight mixed flock, I wouldn't worry about adding Faverolles.