Welcome back to the game!
Yes, an army can be composed of detachment from faction A (Harlequins) and another form faction B (Druhkari) as long as they share a faction keyword (Aeldari).
In the big FAQ that was released, in matched play you can no longer form a detachment using following keywords: Chaos, Imperium, Aeldari, Ynnari or Tyranids, so no mix-and-matching Druhkari & Harlequins in the same detachment. You wouldn't want to do it even if it was allowed as it would prohibit you from accessing chapter traits as the above poster explained.
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