Unbound detachment with a Formation

I want to run my Harlequin formation Cegorach's Revenge (which is my primary detachment) with some of my dark eldar to fill in any weaknesses. If I add Dark Eldar in the list unbound would that make the entire list unbound? Will I loose any buffs to using the Cegorach's Revenge formation?

Formations can be used in Unbound armies and retain their special rules but your army would still be Unbound.

Quote :

Unlike other Detachments, Formations can also be taken as part of Unbound armies. If they are, their units maintain the special rules gained for being part of the Formation.

Thanks