Tchaichovsky! Personally, I like him better than Mozart. His music would fit better too. And he's less used. yeah Beethoven could work well too. But I'd use Tchaichovsky.
Enya could work too... but only to focus on the relationship between Ashitaka and San. As for the song, you should choose that.
Kusoyaro: We don't need a leader. We need to SHUT UP. Make what you want to make, don't make you what you don't want to make. If neither of those applies to you, then you need to SHUT UP MORE.