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...
http://discussions.apple.com/thread.jspa?threadID=2767370&tstart=0
Leopard requirements/10.5.x...
* 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.