Running macOS Sierra (10.12) or macOS High Sierra (10.13) on a 21.5" mid 2017 iMac I'm looking at buying a new 21.5" mid 2017 iMac which will ship with macOS Mojave 10.14.

I have some software that will only run on macOS Sierra 10.12.

Is it possible to downgrade to macOS Sierra 10.12 on this iMac by wiping the hard disk and reinstalling the OS from a USB installer? Does the mid 2017 iMac have the drivers to run macOS Sierra? If not do they have the drivers to run macOS High Sierra?
The mid 2017 21.5" iMac (both Retina 4K and non-retina models) originally shipped with macOS Sierra 10.12.4. Thus, both macOS Sierra and macOS High Sierra have the drivers to run on either one of the iMac's.

It would be possible to run the last (publicly available) release of both macOS Sierra 10.12.6 and macOS High Sierra 10.13.6 on either one of the iMac's by installing via USB installer or via Internet recovery.

The installer can use downloaded from the Mac App Store:


macOS Sierra 10.12.6 - Mac App Store
macOS High Sierra 10.13.6 - Mac App Store