Doesn't it say somewhere (I can't recall currently at this time), that Legolas was one of the few elves to be born in the third age? (which would mean that he was not born at the time of the Last Alliance)
If Haldir was alive at the time. He most likely would have been there. I could just not see him sitting out of a battle such as that one.
But with Haldir we truthfully just don't know... So if you would like him to have been there he can be
