Hi snes1423,
Even if using a FW drive is tempting to keep things separated (as drive swapping is a pain on the PB 12), it could be easier to start with a partition on the internal drive to make sure everything works as described by the guide for unsupported machines (it can already proved to be tricky).
Then once it works internally, I would think that it is as simple as cloning the internal drive to the FW drive (as Macs have been booting off FW drives since the iMac G3 days).
And if you are reluctant to touch your internal drive, you can also burn the latest unsupported macs os9 disc and do a test run booting off the disc. Then once it boots properly and behaves okay, you can use the os9 installer to format and install on the FW drive.
Have fun and keep us posted of your progress