The fish which I have kept which I have watched closely enough re hierarchies are plecos. I started with zebras. I had to put the proven breeding group I purchased in a species tank. I already had a couple of zebras but was told by the person who was responsible for me getting the new group warned me about this.
She explained that the group I was buying had an established hierarchy. She said if I mixed the 13 coming in with the two I had that this would disrupt the pecking order. The result was that the fighting to establish a new hierarchy would start again as a new pecking order would be needed. She also said that this would stop the spawning until it was settled. So I moved the two I had to another tank. I also learned that it wan only the males which had a hierarchy, but that the female group also had one.Only the top females got to spawn with the top males. There rest of them were out of luck.
The only way pecking orders get worked out is by fighting. Often this results in injuries and sometimes even death. With my zebras I also learned two more things about this. Normally, it is the bigger fish which wins such confrontations. But for zebras it is common that the Alpha male is not the biggest. What this taught me is that when the fish are close in size, attitude can matter.
The second thing I learned has to do with the difference between fish in the wild and the same fish in our tanks. In the wild there is a lot of space. When one fish in a fight is clearly losing it can end things by running away. In the wild there is a lot of space to do this, but in a tank this is the opposite. Often the winner in the fight will chase after the loser. often the goal is to kill it. So, the best way for the loser to prevent this is by being able to get out of the sight of the winner.
The problem is in many tanks it is not so easy to get out of sight. As a result all of my breeding tanks have lots of cover and ways to break up the sight lines. For the plecos I do this by filling tanks with lots of caves and a ton of rocks, slate and wood. If the loser can hide out of site of the winner the fight always ends there.
But I also learned something else. just because a pecking order has been established does not mean it is permanent. Often, a subordinate fish will be growing. It often reaches a point where it would seem if wants to challenge its place in the pecking order. if it has grown bigger and stronger it might be able to leapfrog the position of the fish above it. So, every now and them I would see more fighting. Fortunately, such fights do not usually last very long as the lesser fish knows it has jumped the gun with the challenge and will back off more quickly. But sooner of later number 1 will lose because it got too old and somebody else got strong enough to earn the "crown."
There are all sorts of tricks we can employ in terms of fighting between fish. I kept a pair of P. nicholsi which required that I remove the male when the female was holding because all he wanted to do 24/7 was to spawn. She wanted to be left alone. Then I got my first batch of free swimming fry and learned that inside a day or two they went from being her children to being her lunch. So the next time I stripped her and returned the fry to the tanks and she went elsewhere. Once the fry had some size it was OK to return the adults. But i usually tried to sell, trade of give away the kids to be on the safe side.
What we as fish keepers and breeder need to do is to learn the specific behavioral traits of the fish we are spawning or merely keeping together in the same tank. We need to learn what will not work and then how to deal with that. For me and the plecos the solutions was to put as many caves, logs, rocks etc, as I could into the tank. For other species I needed to find different solutions.
One thing we must learns is that how the fish might behave in the wild is often not the way they will deal with things when confined in a glass box. Keeping fish is a never ending learning process. That is part of what makes if fun, IMO.
(edited because nobody can outspell that 12 year old who won the Bee, but nobody can out typo me)
