I don't think the even or odd number thing matters much. I started with 6 Red Bellys and now currently have five ( They killed there brother ). Aggression can be calmed by keeping the water temperature lower ( under 78 degrees ) and by keeping them well fed. I have found with mine as they get older they tend to be less and less skittish. Piranha-Fury has tons of Information on all the diffrent types of Piranhas. Advice : If I could do it again I would go for the Caribe, they are more aggressive and they get bigger then the regular Pygos. Good Luck