> Early MacMini G4 computers does not support units more than 128 GB Units

This is certainly not true. The original Mac mini G4 (1.25 or 1.42 GHz, 32 MiB VRAM) is not more limited in this regard than "silent upgrade" Mac mini G4 (1.33 GHz with 32 MiB VRAM, or 1.5 GHz with 64 MiB VRAM).

> I have MacMini G4 clocked at 1,415 GHz, am I out of danger and can install
> any kind of hard disks, or am I stuck to a limit of 128GB hard disks?

There is no such limit with any Mac mini G4.

> Does this limit afflicts only MacOS and I can format, partitioning and use external 1TB 3,5"
> unit and internal 320 GB Hard Disk from MorphOS, or is it a limit of the IDE electronics?

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=7586&forum=9&start=7

> Should I be able to reserve space for N number of MacOS partitions MAX of 128 GB
> and the system will see it with no problems?

Yes, you should be able to do this, but there is no need to restrict MacOS partitions to 128 GiB. Filesystem for MacOS system partition should be HFS+, which MorphOS has read support for.