Hi Joe, and a warm welcome to the forums & Macs!
Apple doesn't advertise it, but if you call them you're likely to be able to buy a copy of Leopard/10.5, but I heard it was in short supply...
* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
minimum system requirements
* 512MB of memory (I say 2GB at least)
* DVD drive for installation
* 9GB of available disk space (I say 30GB at least)
You have to call Apple & likely ask for a Product Specialist to get it, if they still have it! Helps to tell them you have an iPad/iPhone & you can't run 10.6.
Not sure it'll work, but you might also try turning on Printer Sharing on the Mini, then use a Generic Postscript driver on the iMac G5.
I recently bought the HP Officejet Pro 8500A Plus (A910g) and I have an older iMac G4 with OS X 10.4.11.
I took a chance and installed the HP drivers for the previous 8500 model (A909) because there is a 10.4 & 10.5 version. The automatic install of the A910 fails because the older driver does not recognize the new printer (A910); however, a manual install afterwards and selecting the A909 driver enabled me to print to my new A910 from my old iMac G4 800MHz. Scanning did not work, but I still have a few things to try out. Besides, the nice scan-2-email and scan-2-network-drive features almost makes that a non-issue.