The very first lyric of the song "Fighting Gold" goes like this:
And everything makes sense except for the fact that the singer reads 運命 as さだめ. Why?
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In Japanese, especially in creative writing (stories, poetry, lyrics, etc), people often specify a different reading of kanji than is conventional or "correct". They do this for various reasons, the most common I believe is to make one word have double-meaning, although the two meanings referred are usually quite similar to begin with. For example, in this case, 運命 and さだめ already have similar meanings but have subtle differences in both their definition and the nuance. And especially in lyrics, the words can have different lengths and/or rhymes and/or sounds that may or may not fit the tune. I am not familiar with this particular song but the changing of the reading in lyrics is very common so this is no surprise. By doing this, the song will be sung as さだめ but it conveys to the reader that this word also carries with it the definition and nuance of 運命. Note that this "trick" only works when the written lyric is also seen, hence if you only hear this song, the listener will not get it. I am not aware of a name to this "trick" but we usually refer to it as 〇〇と書いて〇〇と読む (in this case, うんめいと書いてさだめと読む).
So basically this is the result of the creative work of the person who wrote this song/lyric and there is no right or wrong. Based on this, I guess a songwriter could put any combination of kanji and reading, but if is nonsense, then it's nonsense and so is the song, and that's that. I once came across a song that wrote 天使 and read it as あくま but that was okay because the song was describing a person who appeared to act as an angel but had evil intentions. If done well it's a very smart play on words and also efficient to make one word mean two. It's all part of the creative process and we read as so, and enjoy it as so.
By the way, if I saw any of this outside of the context of creative/artistic writing, then it would simply be wrong and incomprehensible.
Your question is about song lyrics.
Same goes with English songs; the wording or the reading does not follow the "ordinary" rules.
According to here:
３ 運命。 宿命。「人には人それぞれの定めがある」
So the most appropriate word is 定め｛さだめ｝. I think the singer must've taken the wording to make the lyric rhyme sound well.
Just to add, Weblio's Japanese-Japanese dictionary doesn't define the reading of the word 運命 as さだめ。
But it says the meaning of 運命 is as
Controlled by the supernatural power, the fate or doom that would visit a human's life. The fate or destiny of a human's life predetermined by Heaven's Order.
If you are taking recruiting test by companies about the reading of the Japanese 運命, and then if you answer the reading as さだめ, I swear to God you would fail.