Firstly by no way you can stay with default FFS. Although HDToolbox shows you 8GB, FFS can only access the first 4GB of it. If you create a partition in the second half of the 8GB, it will appear to work at first but actually it shares its space with other partitions inside the first half. These partitions will overwrite each other. If you write a file to one partition it will cause corruption on another partition and vice versa. So FFS <= V40 is a no-go unless you carefully keep everything inside the first 4GB of the drive.
To use the full 8GB you have to use a file system which uses HD_SCSICMD a.k.a. Direct-SCSI. Your choices are PFS3ds, PFS3aio, SFS <= V1.84 or FFS V44.
In order to use more than 8GB you have to replace scsi.device by a new version. Because scsi.device is hard-coded in the Kickstart ROM, the only way to replace it by software is to load it into memory and reboot to activate it. This means the first boot will last a lot longer because all the hardware is enumerated twice.
Regarding HDToolbox, it will do everything right for you, as long as you ignore the numbers it shows. It will only ever show numbers below 4GB. So a 15.9 GB partition will be shown as 3.9GB and a 16.1 GB partition will be shown as 0.1 GB. But this is only the GUI. The partitions themselves are correct.
FixHDDSize can only work with a new scsi.device. 8GB is not a limit of HDToolbox but of scsi.device and thus also limits FixHDDSize.
