I would put the 300 GB monster into an external NAS enclosure and use it for data to be shared among the PCs at your home.
For the Amiga I'd use a 8 or 16 GB CF card which has plenty of space for everything you want to do.
Regarding partitioning, every partition which should boot the Amiga must be inside the first 4GB of the drive. So either you have all operating systems in the first 4GB (which would mean ca. 1 GB for each OS) or you make a partition with OS 3.x which can load the OS4 Kickstart and the Linux and MorphOS kernels.
Either way, you have plenty space. Even 1GB for each OS is already plenty.