Uhh... I tried to add myself as the artist using artist:furx in this piece https://e621.net/post/show/89062/ (to get rid of the unknown_artist tag) but that changes me to artist in every other piece that features my character. But for the most part I'm just the commissioner of those pieces, not the artist.

What should be the correct way to tag this one piece which I did actually draw myself without screwing up the tags in the other pics that have been uploaded?

Use suffixes.

Apply them as needed. Clean up the current character tag with the suffixed version. I've gone ahead and cleaned up the posts with the character tag into their respective places.

All right, thanks!