The songs that I managed to put harmonies still have harmonies after deleting the song cache and they work normally. None of the 2010 dlc harmonies work. I added the harmonies pack to my xbox by following old youtube video about this topic.
You are about to leave Rhythm Gaming World and access the external songs DB. While no song or file sold for commercially available videogames is hosted there and only fan generated content is allowed, Rhythm Gaming World has no direct control over the database and no files are hosted here. Alright, we had to made you aware of this, now you can visit the database!