I don't think I have ever shrunk a partition except C drive in WinNose but a
friend of mine does it all the time I think just using GParted. And I think
the Live installer may give you that option.
Re the logical partition:
I must have made the 300 GB logical partition first thing, then I can click
on it and find a 'create' or 'add' option to carve out a slice for the /
mount point for each OS during their install.
OS1 is Trisquel 8, OS2 is Fedora 25, And, just installed, OS3 is OpenSuse
42.something Leap. I'm on a roll now!
I have about 90 GB free space and an available primary partition that might
go to some kind of BSD but grub won't see it I guess so I'll have to put that
boot loader on a stick or CD.
