I know this is very late, but I think very important..
I needed to install final cut pro 6 or 7 on my 2013 retina macbook pro laptop…Of course it was using APFS, many many problems, but I finally was able to figure out the solution.
The solution was not elegant and took a total of 3 days, and much much swearing, but it worked, I have 2 partitions, the minimum number required, one is HFS and the other APFS…
This is very very important, you need to wipe the hard drive clean, format back to HFS [journaled extended] 1 partition, then reboot using the internet installer, DO NOT USE any boot USB, it will not work…
I was able through the internet take the laptop back to base install, Mavericks, which is not listed in the APP STORE, for some reason the internet installer which is very slow, on my connection took about 12 hours to install, used Mavericks.
Once I was able to log into Mavericks, I created a 2nd partition called it High Sierra, formatted to HFS [journaled extended]
I was able to install final cut pro, I needed to first install Rosetta off the Snow Leopard install disk. Then I opened the FCP launcher, and installed only FCP7, it installs compressor and qmaster, this is not optional…
Then after upgrades and such, rebooted holding down the option key after the chime, and was able through the internet launcher install High Sierra onto the 2nd partition.
I was worried that APFS would destroy the HFS Mavericks, as according to Apple info this is the case, it did not for me, so once I was able to log into High Sierra and updated High Sierra I installed FCPX.
So now I have 2 versions of final cut, 1-FCP7 on a partition called Mavericks, and 2-FCPX on a partition called High Sierra, both are 50% of the inbuilt hard drive and so far no errors!!!
At this point no plans to change, but will look into a 3rd partition on the High Sierra partition later if needed..
STEPS..
1-Wipe hard drive,
2-create a single partition formatted journaled extended
3-use the internet installer to install the OS suggested.
4-once installed and updated, create a 2nd partition, formatted to journaled extended
5-Install Final Cut Pro 6 or 7, you might need Rosetta from Snow Leopard
6- restart using command+r
7-install the suggested OS
8-upate and then install zipped or downloaded copy of fcp x
I tried to make a usb installer, but it did not work…your skills might be better than mine…