Some wondrous seven string players:

George Van Epps. The chord melody master played a custom Epiphone and then a production Gretsch. Believe he tuned the extra bass string down to A.

Bucky and John Pizzarelli. Think they may still use Gretsches.

Don't forget Lennie Breau. He used a high A string. Think his classical was custom made.

Too hard for me to contemplate. Six strings are confusing enough!