Is there any inherent issues I would face if I was to network (printer and files) between different versions of Mac OS - Leopard, Tiger, Panther and OS 9?
OS X has had Bonjour since 10.2. It's software that lets devices discover each other on the network, so networking different versions of OS X should be fine (although a bit clunky pre-Leopard in my opinion).
I have no experience with OS9 so I can't give any advice on that.