So yeah, I don’t think it’s wise to share partitions between distros anymore, except for swap, just too many headaches when they don’t agree, especially when an OS’s /etc/fstab file deals with partition UUIDs instead of pathnames… So I just reformatted the entire disk, I now have:
/dev/sda1 : /boot : 2 GB, exclusive to Linux Mint.
/dev/sda2 : swap : 2 GB, for anyone.
/dev/sda3 : extended : 100 GB, five 20 GB logical partitions with one OS in each.
/dev/sda5 : ext4 : 20 GB, Linux Mint.
/dev/sda6 : ext3 : 20 GB, Slackware.
/dev/sda7 : ext3 : 20 GB, unused.
/dev/sda8 : ext3 : 20 GB, unused.
/dev/sda9 : ext3 : 20 GB, unused.
/dev/sda4 : ext3 : rest, for Linux Mint, but mounted by other systems and put in their /etc/fstab as necessary.
After yet another failed attempt at installing Debian I tried my hand at Slackware, having been too intimidated and lazy to attempt the CLI-based install. Luckily, the only command-line operations you actually have to perform are partitioning the disk, which I had already done when installing Linux Mint. The Slackware install is actually a breath of fresh air. Can’t connect to your college’s wireless Internet? Slackware doesn’t care (unlike Debian). You only want to install selected packages? Go ahead! You don’t want some programs running at startup? You’re the boss! It’s like a pleasant midpoint between overwhelming configuration (Gentoo, Arch Linux, LFS) and exceedingly little (*buntus). It’s like a lovely development suite you can get up and running on a Sunday evening, when your brain’s fried and you just want to have some light fun (in the form of coding, no less).