Big Jim,

On the poem and the lyric, master songwriter Sammy Cahn once wrote in his classic book The Songwriters Rhyming Dictionary;

"What's the difference between a poem and a lyric? My answer is that a poem is meant for the eye, while a lyric is meant for the ear, but both reach the mind and touch the heart."

So you're right, the lyricist must know how the words can be set to music. I don't know if they are poets if they don't, though :-)

The difference Cahn is talking about is about singability, and I would say also knowledge of musical genres come in handy today. Some things works well in one genre, but not at all in another.