I have a late 2014 27-inch 5K Retina iMac, with a "Fusion Drive" composed of 1TB HDD and a 120GB SSD. This particular Mac has Yosemite (10.10.5) installed.
In order to do some testing of High Sierra, I wanted to create a new HFS+ Journaled partition using 200GB of the available 400GB, and once I install a bootable High Sierra, let High Sierra convert its installed partition to APFS.
Can I do that? Can the Fusion Drive setup have one Yosemite HFS+ Journaled boot partition, and one High Sierra APFS boot partition?