Chords are tonal combinations(Harmony), and the lead is a tonal line vivified by rhythmic beat(Melody).
So, it is important to know the compatibility between chords and scales. However, You do not HAVE to follow one scale in order to compose a song, as long as you know the Scale Relationships, you are going to be able to use the relative scales or parallel scales.
Just take it slowly and start studying the harmony of the Major Scale(triads, inversions, extensions). Then you can study the harmony of the Harmonic Minor Scale, and so on.
Soon everything is going to make sense.
Hope that helps.
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