Elven Lineages

[Legolas]

From the appendix of The Silmarillion:

Alone among the Noldorin princes he [Finarfin] and his descendants had golden hair, derived from his mother Indis, who was a Vanyarin Elf.

Of Fingolfin's house, only Idril Celebrindal, Turgon's daughter had golden hair, and, given the quote above, she must have received it from her mother, Elenwë.

Besides, all of the lines of the House of Fingolfin are complete - they ended with Gil-galad, Eldarion and his sisters, Elrohir, Elladan, and Maeglin. I don't see anywhere for him to fit in. He was the Chief of his own house, Chief of Gondolin’s House of the Golden Flower. Given that he slew a balrog and died himself at the time of the Fall of Gondolin (510 FA), he was probably born sometime between Fingolfin himself (perhaps before) and Maeglin's birth in 320 FA to be old enough to accomplish such a feat. The family tree looks quite complete.

The Encyclopedia of Arda asserts that it's possible too, but I don't see how.