You are more safe starting out with the Citadel Journal rules provided by Games Workshop. The EO Revision is meant as a sort of patch to fix some holes in that original list and make Harlequins more fun and reasonable to play. However, not everyone will accept the Revision as legal. Thus, it is safer to at least familiiarize your opponents with the original.
Eldar can't take Harlequin detachments. A Dark Eldar, Eldar, or Exodite list can take Harlequins from any section as Elites up to the maximum allowed on the Force Organization chart. See pg. 5 in the CJ39 list for details.