YUP! gotta seperate them. The pleco might not be interested in eating the SN, but it will suck off all the slime coat and stress out the SN. Plus, if you allow that, what if the pleco wounds it one day? Not worth taking the chance. I've seen Chinese Algae Eaters suck the slime coat off my old LSN, it did no harm but the LSN hated it. SN's are easily freightened, very aware of their surroundings.