UPDATE
In version 2016R1 we've added a new CompressionLevel
property into ftp.Settings
so that you are able to change the Compression level used in mode Z.
Download the free trial of Rebex FTP/SSL
Please note that the setting will only be effective for uploading files, as the compression level used when downloading is determined by the FTP server.
As to your other question - if you turn on mode Z, and the FTP server does not support it, the Rebex FTP client will automatically fallback to mode S (FtpTransferMode.Stream). We have improved the FTP Mode Z documentation on our features page - thanks! This being said, setting the Zlib mode is safe like this whether the server supports it or not:
Ftp ftp = new Ftp();
ftp.TransferMode = FtpTransferMode.Zlib;