Montunoman, when you work in scale mode you have to set the scale to each song.
Depends on what you use, with voiceworks it takes me 3sec to set it up.About how if song changes scales or key. You can do that to with voiceworks pretty easy in song mode you can set it up. I try not to over use harmonies because they start to sound too dull and boring after a while.
The way I do it is, I have 4 or 5 presets. On each preset I have set up the scale for upper 3rd, or lower or upper 5th, or lower octave from my voice. With voiceworks I can choose instantly which voice is going to do harmonies. It's pretty easy with voicelive 2-3 with steps to do the same..I do have to have in my harmonizer ability to do custom scale. That way you can choose which harmony sound when. Lets say you sing in C major and set it to 3rd up which is pretty easy, but also you want to add 5th only when you sing lets say G note. That is why I like custom scales.