JD, If anything I think aggression between the pleuros would come from being a possible "threat" to one another, whether it's competing for food, claiming territory, or perhaps possibly a building bond between a pair (if housing more than just 2 pleuros?),is it more of an issue of aggression between different sizes or, if the size difference is great, cannabalisim?
Pleuros never bothered or tried eating their little tankmates, which were smaller albino IR sharks, and small clown loaches. You can see in vid the pleuros go towards them, but then just back off.