Those are, coincidentally, species often found in groups. Do you think that has something to do with it?
Could be......maybe less squabbling between all of them before the pairs sort things out.
Semis have a group dynamic, but not as clear cut as anthias do.
I was lucky with the first 4 as I got two pairs out of them and one pair was dominant towards the other. I had both pairs seperated for a time and then when I moved them to a 180g they were all fine together.
I later added two more smaller ones that were paired and when I got them in my tank, one turned on the other. I had to remove one and went quite some time with 5.
A few years later the one that was bullied and removed was paired with one of my older ones and they got along fine.
Using mature fish is easier, because you'll know more whether they are male/female. With juvies, two may get along and then when maturity kicks in you find out they aren't compatible. That's one of the isssues now.................I don't see larger specimens come in anymore.
Back then it was common to see two 7-8 inch specimans for sale. They are so beautiful when big like that. because their bodies round out more and take on a saucer shape.
On the pkastanis, I think it helped the tank was croweded. I put in six 4-5" adults with my Semis, a Purple Tang,and some small dither fish.
Who knows, the tang may have helped as he was the boss.