Hi Wendy,
Our company bought a Mac (Retina 5K, 27-inch, 2017) for us that came with High Sierra.
There was no way for us to use Final Cut 7 anymore and we do have a license for Final Cut X too but as you might know FCP7 is a profesional software and the other one is just a “iMovie with steroids”. The first one has everything you need and the other one… OMG, it is just annoying.
We all still need Final Cut 8!!! Anyway…
I am usually a Windows user but I was able to do all these so it cannot be that difficult.
This is what I did:
Download Mac Sierra from Apple.
Instal Sierra from USB first in a partition and worked, later I tried deleting everything and I was able to downgrade from High Sierra to Sierra. Not sure if I have been lucky but I am using Sierra 10.12.6 though my iMac came with High Sierra. FCP7 works.
…And I am so happy now.
I hope this gives you hope. And I wish you all good luck.