Can I Use GarageBand to set up my tracks for Live Tracks for Sunday Keys?

You can use GarageBand if you'd like, but you won't be able to add arrangement markers for Live Tracks to reference. Ableton Live can be used for composition, but historically MainStage doesn't sync as reliably to files bounced out of non-Apple DAWs, even if the file format is still .aif or .caf.