as for physical HDs, we do not know of any models which would not work with OS9 when using firewire or SATA.
but pls search the forums for protools´ strange partitions size limit. not sure if it applies to all versions - but protools is using a custom disk access method and only supports writing to partitions up to 178 gb or something.
so if recording audio with PT is the main reason why you add these drives you might consider other solutions or at least format likewise.
replacing the internals drives in a quicksilver is doable - there are other models where it is far more complicated.
what works great is to use some cheap (3 euro) IDE to SATA adapters, and then remove the steel trays where the disks are in so that you can place the new disks horizontally on the floor, with the SATA adapter directly mounted to the disk.
you might want to turn off the auto-reboot function in your OSes before you unplug the machine from the net.
other than that it is opening the case, locating the disk, and removing 2-3 screws.
if you are paranoid, use latex gloves and put a towel over the board in order not to break something.
to partition, especially firewire, best option is to use 10.4.
in 10.5 you cannot install OS9 drivers and in OS9 you need third party software. turn journalling off, even if it is going to be used in OSX , too.