Mmmm - I'd thought bootmagic had died and gone to hell.
Guess not - each man to his own poison.
I'd suggest you pre-allocate the FAT32 "shared" partition in advance - a good
strategy IMHO. Safest way to share data.
Mepis uses a GUI installer - not something I'm generally in favour of. It should allow you to not install a bootloader, but I honestly can't remember. Your best option would be to choose to install it to the root partition, and load it from bootmagic - means 2 loaders, but if that's what you want ...
Do the partitioning from the installer - tell it to use the unallocated space, and it'll give you a separate /home and do reasonable general setup. Your personal files will go in /home - handy if someday you need to do a full re-install. The installer will offer to keep that partition rather than over-write it - yes, I've "been there, done that"