Would depend on the severums attitude when it gets bigger. I have roktiel in my tank with them too, he is a inch smaller. He is the most passive so he gets chased by everyone. Never seen my nic do any fin damage to my other fishes though. I have heard their mouths are too small to do much damage. I would just let them settle stuff out for a week or two and see how it goes. If the nic starts doing some real damage, I would take him back or seperate him.