Don, you are using scale mode. That is why it sounds good in this song. It doesn't matter if it's a major, minor or custom scale. It's still scale mode as you described it.You said you set up the key you play in. The key with harmony units, Digitech ot TC-Helicon is not to over use them. Me personally don't like to use more than 2 harmony voices at once.Actually, I will usually only use one voice for harmony. Or pan harmonies left and right.

Harmonies in this song sound nice and I wouldn't change anything.