#1: Standard minidiscs hold ≈150MB, last time I checked.
#2: (to petals) Are you talking about an extra 70MB leftover on a disc that you want to burn as a session, or are you talking about an actual 70MB disc (does such a disc even exist?)? If you're planning to burn the .mp3 files for data purposes (i.e. if the disc isn't going to be played in an mp3 CD music player), then you can burn the files to the disc as data .mp3 files using Toast with compression. (You know Toast can compress up to 1GB on a regular 80min. CD-RW, right?)
The first solution that came to my mind was reencode the songs that need compression and select a lower bitrate so you can keep the files compressed. Make sure you encode form the original AIFF source, NOT the .mp3 files (unless you absolutely have to), and I recommend setting the bitrate setting to two options lower than what each of the .mp3 files are encoded at (i.e. if they're encoded now at 192Kbps, reencode to 128Kbps; if 320Kbps now, reencode to 224Kbps). Make sure you have the VBR setting in iTunes set to max if your bitrates get low.