We have seen a few other El-Aurians, most prominently [Soran][1]:

![Soran doesn't wear a hat][2]

Also, we have seen another male El-Aurian: [Martus Mazur][3].

![Martus Mazur without hat][4]

Also from *Generations*, when we meet the lovely Mr. Soran, we also see [a lot of Guinan's people in one of their space stations](http://en.memory-alpha.org/wiki/Unnamed_El-Aurians). Female El-Aurians sometimes wear hats:

![Unnamed El-Aurian with hat][5]

And sometimes do not:

![Unnamed El-Aurian without hat][6]

And the hat of the first one is quite different from Guinan's. Since Guinan seems to wear civilian clothing (not a starfleet uniform), it stands to reason the hats are just her personal taste.

  [1]: http://en.memory-alpha.org/wiki/Tolian_Soran
  [2]: https://i.sstatic.net/gTyfRm.jpg
  [3]: http://en.memory-alpha.org/wiki/Martus_Mazur
  [4]: https://i.sstatic.net/ebU0Rm.jpg
  [5]: https://i.sstatic.net/BwGY4.jpg
  [6]: https://i.sstatic.net/lqyg7.jpg