I found somthing intresting Regaurding harlequin formations in the book.

[Xaric]

Well from what people have said online harlequins are very limited to there selection's due to the FOC and formations provided or loose out on the rules that give a huge benefit to them aka run and charge turn two so I looked at it differently instead of looking at each model as a choice like troop elite fast attack and heavy support how about making the units in the FOC/formation as those choice's. From what the rule book clarifies you can use a formation instead of the FOC as a additional detachment so far with my research please object if you find a rule that restricts this.

HQ formation choice's
1 Large Scale Formation is the Cegorach's Revenge this would most likely be used for apoc sized games of 2000+
2 Medium Scale Formation is the Harlequin Masque this would be most likely used for normal games of 1500+
3 Small Scale Formation is the Serpent's Brood this would be most likely used for small games of 750 - 1500
As you can tell by each formation they come with Emissary of Cegorach making them ideal as your primary detachment.

Allyed supporting formations choice's

Cegorach's Jest this unit is to add more troops to the field or expendable body's to take objectives or divert the fire to them I would most likely class this formation as the troop/elite choice formation good with any HQ formation choice because of the low point's.
Troop make them cheep and high on numbers.
Elite make them low numbers but expensive.

Cast of Players this unit is more of a Elite/fast attack choice giving all models that are of the eldar or dark eldar within 6" crusader making them hit fast and harder the elite is good for any formation due to more strike power. fast attack is more restricted to the harlequin masque or the serpent's brood because the cegorach's revenge will most likely have models too spread apart to benefit from the rule.
Elite make them expensive with low numbers to deal a dent in armour units or kill mass horde's.
Fast attack make them cheep but high numbers to provide too many targets for the enemy to deal with.

The Heroes' path a sub HQ formation the reason for it they don't really benefit your primary detachment in the such as taking or giving buffs to them because all there rules effect really only the models in the formation but they can act as how you would say a HQ unit best to be taken with The serpent's Brood due to there low points cost but if you have models spare you can use them in the Harlequin Masque too unfortunately you cant use them with the cegorach's revenge due to you cant have more then 1 solitaire.

Faolchu's Blade this is your go to heavy support or fast attack formation depending on how you build it but can destroy a lot with its special rule keeping them alive.

Heavy support maximise your voidweavers and keep the skyweavers low model count.
Fast attack maximise your skyweavers and keep the voidweaver on its own to provide supportive fire.

Now the idea and reasoning behind how I worked this out I was watching a playwright on youtube how the actors all worked together with there rolls to preform a play with no hiccups or mishaps even though it does happen makes each person as important as the person who choreographing them.
My second hint to confirm how I viewed it looking at most of my codex I have on hand there troop choice is the only one I can see that can have all the same wargear as there leader and everyone has a +2 look out sir it looks as if they was intended to make people try to look at how to build a army differently.

Because of this way of how I see things now I will most likely take the following formation in my army
the Serpent's Brood as my HQ unit acting more as a troop choice
2x Cast of Players as my Elite/Fast attack unit
1x The Heroes' path as Sub HQ unit
1x Faolchu's Blade as Heavy Support unit