My suggestion is to make your playlists within iTunes, then use the iTunes sync manager or import those specific playlists into rekordbox rather than importing your entire iTunes library. That allows you to keep things clean and maintain a smaller rekordbox collection.
As for the tagging, the majority of the tags are stored on the file itself, so changed tags can be read by any application.
The iTunes Grouping conversion is because the Grouping tag is not used by rekordbox, so it allows you to convert it to the Label tag within rekordbox.