Also note, if you upgrade your libraries to a newer system, such as an Apple Photos library, only the raw photos will be readable by the older system, and not the libraries, tags, albums and favorites.
If you are just going to flip booting back and forth, only the first step is needed, as the Option key booting will choose the operating system that is loaded regardless of the drive formatting.
And an extra step to make 10.13 or later readable to the 10.12.6 booted system would be to clone backup the newer system, and wipe and reformat the newer system's drive as HFS Extended Journaled. An extra step to make 10.13 or later readable from 10.12.6 or older systems would be to make sure the 10.12.6 or older system is to wipe and formatted HFS Extended Journaled before installing 10.12.6 or earlier. With 10.13's release Apple introduced APFS, which is not readable by older operating systems.
