you can't boot the powerbook off of those discs and install directly (it will say the hardware is unsupported).
however, you can connect the powerbook to the mac mini via firewire, boot the mini from the install discs, boot the powerbook in target disc mode (holding down T while you turn it on) and then install onto the PB's hard drive from the mini.
After the first part of the installation, the computer will restart and the mini will actually boot from the PB's hard drive, so it may at first appear that you've reformatted/installed on the mini's internal drive by mistake. however, once you reboot again, it will start from the internal drive and all will be well. I recently did this with iMac discs onto a PowerBook.