When kept in small groups my experience is that poly's are somewhat territorial especially when they are around the same size, I've kept ornates, teugs and palmas; and every time the fish grew to a similar size in small groups (2~3) in a planted aquarium or otherwise with hides there has been territorial bouts for the best spots, food, etc. But then when the size difference is large both fish tolerate each others presence.
My guess would be that poly's in the wild tolerate smaller polys within their territory as long as there is no doubt who is the biggest baddest mcdaddy around, but as they grow up to a similar size they start to tussle for the belt and the losers disperse further into their watery world.