Single User Mode will get you around the password/admin rights problem(http://osxdaily.com/2011/04/25/change-admin-password-mac/), but
I'm more concerned by the mention of PowerPC architecture. (I thought Leopard was the last to run on PowerPC?)
Not sure if Rufus can handle the OSX installer, but there are steps on Apple's site for creating using just the installer(should be in the App Store on that machine) and Terminal: https://support.apple.com/en-ca/HT201372
Download the macOS installer from the Mac App Store. Quit the installer if it opens automatically after downloading.
The installer will be in your Applications folder.
Mount your USB flash drive or other volume. You could also use a secondary internal partition.
Open the Terminal app, which is in the Utilities folder of your Applications folder.
Use the createinstallmedia command in Terminal to create the bootable installer.
For detailed usage instructions, make sure that the appropriate macOS installer is in your Applications folder, then enter one of the following paths in Terminal:
(this is where yours will be different:)
/Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia
Example for Mavericks:
sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Mavericks.app
Replace the Volume paths and the Mavericks.app with the right ones in your case...should work with the older versions if you have to go back some more.
Another option (since you don't seem to need anything specific to OSX) would be a PowerPC-friendly Linux distro. Can't post another link yet, but go to Distrowatch.com's search and check the PowerPC architecture option. Ubuntu and it's variants are probably easiest to learn on. If nothing else, you'd still be able to get the latest versions of software this way(chrome/firefox/python/etc).