Question on Mohave and High Sierra working independantly

Please do not do again whatever you did to get that horrible shrunken picture of text instead of actual text, as you posted above. Your post is very hard to read, can not be displayed larger as is my preference, and can not be auto-translated because it is not TEXT.

Yes you can do what you described.

My Mac Pro 5,1 has a drive with 10.12 Sierra (the last MacOS that uses MacOS HFS+ extended format on the boot drive by default), a drive with 10.14 Mojave that uses Apple File System (APFS), and three other drives with different versions of MacOS.

When you install Mojave 'live' it will insist on a Firmware update. That is a good thing to do, as it provides additional capabilities.

That firmware is fully backward and forward compatible, and is NOT stored on any drive, it goes into a special firmware store on the processor card. So its update will become a 'permanent' addition to your Mac, even if you throw the drives away.

if you run into any trouble, Readers are eager to provide additional help.

Mac File systems

Backward compatibility is excellent. MacOS versions AFTER Sierra to current day can read and write older Mac HFS+ extended format drives with Zero issues.

Forward compatibility is not quite possible. When a MacOS High Sierra or older Mac tries to look at an APFS drive directly, it sees an APFS container disk, that is unknowable. it does not see any directories or files.

One way to launder out that incompatibility is to connect across a network. The nuances of drive format are not visible on network drives. As long as the Hosting Mac can read and write its own drives, so can any Mac that connects to it across the network, older or newer.