OP wrote: "Yes thats the problem, the new file system don't work on the old hdd. Not yet anyway."
No, that's NOT "the problem".
I have a late-2012 Mini. I installed High Sierra on an OLD firewire800 external hard drive.
The install went ok and the drive boots and runs High Sierra without problems, except for the fact that the boot is SLOW because of the old drive platform.
But once up-and-running, it runs well enough.
You should have no problems, IF you take the extra steps to create a bootable USB flash drive installer with High Sierra, and then boot and install that way.
I would also STRONGLY SUGGEST that you use either CarbonCopyCloner or SuperDuper to create a bootable cloned backup of your current installation on an external drive.
This way, you'll have a "pathway back" if the OS install doesn't go as you want.
Without doing this, you could be "up the creek without a paddle".
A Time Machine backup... well... that ain't going to be the best way.
If you install High Sierra onto the SSD, I'm going to -guess- that it will get automatically converted to APFS unless you take steps to stop the auto-conversion from happening. Same as with a factory-installed Apple SSD.
There IS a way to prevent this, but it involves using terminal early during the install process.