slideshow music

I am using GarageBand to create the sound track for a video or slideshow.

When you export your slideshow as a video, you can add this video to a movie track in GarageBand. Than you can mix a new soundtrack with clips from your playlist freely in Garageband and save the mixed soundtrack as a new song and use the mixed song with your slideshow.

You have to add each song to the slideshow you want. I don't thing you can add a pre prepared playlist, and it will always use the full song length.

if you want to use shorter clips from songs, you might want to try using an external sound editor to create the sound track. EG audacity.