Is it OK to use an HFS+ start disk with High Sierra? I have a clone of my internal SSD on an HFS+ partition of an external drive. I was wondering if there are issues with using such a disk as a startup disk with High Sierra.
Yes - High Sierra fully supports running / booting from HFS+ since it doesn’t even change any system over to APFS except for Apple SSD based systems. Fusion drives and spinning HDD hard drives all remain on HFS+ as do all external drives connected to your High Sierra system.