Oto's do like to be in groups, my brother has three and they are fairly independant but when they see a fellow oto they tend to follow them around. I think one follow's another in search of food, when they find it they will graze on the food together.
My advice would be. Make sure there is enough algae for more than one oto to graze on, they die regularly through lack of food and a happy oto is a fat oto.
They won't affect your bio load too much but just make sure there is enough food to go around.