Personally I think Huxley and Salinger are a little too strongly associated with their famous bearers. Hux and Sal are cool nicknames, and they go well with Zelda - albeit creating a strong theme for future children - but expect to get a lot of comments on the names. I think all the other choices are flawless, and even if you do choose the more "out there" Salinger or Huxley, they're still very nice names :)
Atticus or Magnus would be my top choices for Zelda's sibling.
Unwittingly, I caused some confusion. My dear husband isn't named Dane, he is Dane - comes from Denmark:D. And his name is Viggo. But you suggested me an idea surely worth consideration this way - maybe is Dane as a mn a good and simple way to honor scandinavian gfathers?
Oh, what I wanted to add writing this post our last name starts with M and ends with -sen, like actors' Viggo Mortensen and Mads Mikkelsen.
Hugo would be gorgeous with Zelda. My next choice would be the spunky Gideon. Salinger, Huxley and Atticus would be too "literary" for my taste when you already have an F. Scott Fitzgerald-inspired older daughter. I'm not sure if Magnus would be ideal for a fn when your "M" surname ends in a "sen".
Magnus- This wouldn't go the best with M-sen as a surname, imho. Also, it always brings something sexual to mind (Magnum).
Atticus- This would be pretty great with Zelda.
Gideon- Ok, but I don't think it goes as well with Zelda. Additionally, if the surname ends with an -n, I'd rule it out.
Huxley- I also love this. Zelda and Huxley (nn Huck, Hux, etc.) would be great.
Salinger- Not a fan.
Hugo- It's ok, but not on par with Atticus and Huxley.
It's a tie for me between Zelda and Atticus, and Zelda and Huxley. Both sets are great.
Hands down, I like Hugo best with Zelda. I think Hugo and Zelda are perfectly balanced together.
What about Hugo Arvid as a combo? (I love Arvid for its Scandinavian flare, its unusualness, and its easy pronounciation!)