As an added bit of info: the Brazilian 7-string guitarists rarely play unamplified. Most have either a built-in mic setup (Rabello) or a pickup (Costa) or both. I have 3 nylon-strung 7-string guitars, and all have some kind of pickup system, from K & K to RMC. The clatter of Brazilian percussion overwhelms any unamplified guitar, and I also play mine in African ensembles and jazz groups, so amplification is, like with steel-string electric guitars, part of the instrument. If you are a soloist, then a well-made concert-quality instrument should suffice in a friendly hall without amplification, but the low notes do benefit from electric reinforcement.
Lester Devoe Flamenco Negra
Lester Devoe Flamenco Blanca
Aparicio Flamenco Blanca with RMC pickup
Bartolex 7-string with RMC pickup
Giannini 7-string with Shadow pickup
Sal Pace 7-string archtop