There is more to it than the number of words or syllables.

In poetry, there is also metre. Each "foot" (equivalent of a bar of music)may be made of 1, 2, 3 or (0ccasionally) more syallables, and the emphasis may be on the first or last, or even one of the others.

The metre must fit the tune, otherwsie you could end up putting all the emphasis on words like "the" and "a" instead of the important words, and the song will limp along.

For a song, it is even more difficult, because certain combinations of sounds are more or less natural to articulate - put simply: some sequences of words are harder to sing than others.

As for one song to the tune of another: this is an important part of the traditional process. Go to the Yorkshire carols to hear pretty much the same words sung to three or four completely different tunes in the space of a couple of hours.