Will the download work on my PSR-SX600?
I no longer own the PSR-S700.
Also, any chance of any download corrupting the instrument's file system?
You don't need an S700 anymore. All available .mfd files can be imported on your SX600 and then used as a Playlist. (The import function will automatically create a Registration Bank file for each record of the imported .mfd file.)
Please, as mentioned, refer to the the SX600 Reference Manual and watch also Casper's video.
Importing .mfd files cannot break anything on the instrument. The only thing that could happen is that an error message appears if a certain .mfd file cannot be used (e.g. because it contains incorrect entries or certain, unsupported special characters).
P.S.
If you haven't done this yet: You can download the SX600 Bonus Playlist (containing 325 song titles) from the SX600 download page (linked above).
Best regards,
Chris