I just added a few new blues to my tank and the first thing one of the hermits that was already in the tank did was attack one of the new guys.
I gave them a blast with a turkey baster, i had to do this a few times to keep them apart.
The next morning the hermit i just added had the shell from the old guy in the tank. They both seemed ok.I think he just liked his shell. You should add a few extra shells into your tank to keep the UFC fights down