This starts an interesting debate.
If as suggested Majestic Angelfish are harem forming with a male holding a large territory, inhabited by a number of females. The male will need to, presumably, physically defend it from other invading males, like in deer, the dominant stag has the largest antlers, holds the best territory and the most females. Then the development of a large operculum spine in the male must be as an intimidation display and only used as a last resort in fights. This would suggest that the spine must develop over time, as the fish becomes more dominant and moves into a better territory. It would also mean that the theory that if/when a male dies one of the females takes over (like anthias) is wrong.
However, if the fish form more 'exclusive' pairs is the spine a display option to attract a female i.e. the female chooses the 'best' male- similar to the behaviour I have seen in wrasse. I had 4 female peacock and one changed into a male and started to court the females who totally rejected him. Another female then changed, even though there was already a male in the tank. The females accepted the new male's fluttering and diving display and that male dug spawning pits which were visited by the 2 females although I never saw any spawning. The other male stayed at the rear of the tank totally ignored by the others.
So does the male spine develop as a masculine intimidation/fighting tool or as a decoration to attract females. If it is the latter then pairing should be easier, I just need to make sure I get a pair with different spine and physical sizes.
If it is the former then I need to be careful that the male with the spine doesn't see the smaller female as a threat to its dominance and potential territory holding and attack it. Unlike on the reef the female can't just swim away and out of sight, it will just be see as a fish trying to test the males dominance and not backing down.