Plecos usualy only fight for space or over food and if provoked too often will attach themselves like a remora to the offending tank mate, rasping a hole in its side with their mouth. Unscaled fish are weak to this form of attack and the damage can be fatal.
If the tank is large enough (if not what are you doing with a pleco in a small tank?, they do get big) construct a hiding place for each fish of equal size/darkness and some hidy holes spare.To each fish a home and if the terrotories build up and then each one has their own. If there arguments then the extra holes provide space to go and hide. Plecos do get more terrotorial as they grow older.
Place food in separate locations to reduce competition at feeding times.