I think Original Artist is generally used for covers and such, i.e. "Who originally performed this song". But you can do whatever you want with it.
How you organize your covers and remixes is up to you, but MusicBee also offers a Remixer tag as part of the multiple artist splitter (click the ... on the artist field).
I tag composers with MusicBrainz' Picard. If you need help getting started with it, I can offer some assistance.