Yes all Midi files follow the GM Standard for compatibility, however once they are loaded into the instrument, you can replace the GM Sounds with the best sounds in the keyboard, before converting to MP3.
As there seems to be some concerns with file size, it may be better to use a WMA file, as at 128k it gives as good quality as MP3 at 160K, but with a smaller file size. (Also, as all Windows systems come with Media Player, which can do the conversion, you will not need any additional software)(But do check recording levels)

Bill