Fingon has long dark hair with braids, woven with gold. Tolkien said so, and I will accept *NO* alternatives to this: Fingon "wore his long dark hair in great plaits braided with gold." Done.
Also, his name almost certainly does not mean hair commander! It is from Findekáno. But more importantly, the 'Fin' in his name designates him as a descendant of Finwë, and has nothing to do with his hair. The best translation I could come up with would be more like 'Finwë's commander'.
Finrod's name is more complicated, I will admit. Whenever you see 'fin' as a name element outside the house of Finwë (ie, in the case of Glorfindel), it is in reference to the blond hair of the elf, and Finrod is of course blond. So, yes, Findaráto
could mean Golden-haired champion (with the blond part understood). Or, more likely, it was meant to be 'Finwë's champion'. I realize that's not how you would put that together typically, but a name of the eldest child of Finarfin recalling his grandfather the king makes more sense then commenting on his hair. Fëanor explicitly named every single one of his sons something-Finwë to make this crystal clear. Fingolfin and Finarfin likely didn't just accidentally slip a 'fin' into their eldest sons' names.
Boromir has his hair shorn about his shoulders. So, anyone with long hair has hair longer than shoulder length, certainly. Jenny Dolfen's idea that Maedhros would have had his hair shorn during his captivity in Angband is a fun one, and I'm fine with adopting it, but we don't have to let that determine all elvish culture hairstyles. We have room for some variety here. It would be good to do some of the same things to Gwindor that we did to Maedhros, so the audience sees his captivity that way. But we don't have to stipulate that *no* other elves can have short hair ever for any reason.
The only way to incorporate buns without being very modern hipster is to go full-on samurai, I think. But Toshiro Mifune died in 1997, so I guess we can't cast him as an elf..... The 'Vikings' hairstyles are very intentionally somewhat modern - remember that part of their goal is to sell to their audience that this ancient history stuff is still topical. You see a lot of modern touches in pseudo-historical costume design to try to make the characters more palatable and relatable to the audience. I'm not saying that nothing is authentic, but rather that authenticity isn't the goal - relating to the audience is. Ask anyone who cares about historical costuming about Belle's yellow dress in the live action Beauty and the Beast if you want to get an earful about this
.