For a HDD which will have both HFS+ and APFS volumes, is it better to format the disk itself in Disk Utility (on High Sierra) as APFS or HFS+?
I have done this by starting with a whole disk APFS partition, I believe. I gave the whole disk to APFS, and then I followed the directions in this article to shrink the APFS partition:
This freed up unused space on the disk, which I think I was able to assign to a new partition in HFS+ format using Disk Utility. I needed to do this because I wanted to have a ZFS partition on the same disk with an APFS boot partition, and ZFS doesn't really interface with APFS (ZFS can replace an HFS+ partition, though).