I haven’t tried this setup per se, but I do know that Apple has been drastically altering the OS ever since they released the ridiculous “upgrade” that was Yosemite. Now with High Sierra, even the file system has been changed. My opinion is this: even if you were to be successful at installing it and then launching it (which I really doubt), you WILL inevitably run into crashes and errors. I ran into crashes just by running Cubase 9.5 on Mavericks (which is generally super stable, but this version of Cubase isn’t optimised for it, and neither is 7.5 for High Sierra). My suggestion, I’m afraid, would be to upgrade Cubase since you generally cannot downgrade to a lower OS than the one that originally comes with your Mac model.