You should have no problems networking those computers. The networking stuff hasn't changed significantly between OS 8 and OS 9. The only thing that may be different is the network adapter in the older Power Mac may only be capable of 10 MB speed, whereas the iMacs can do 100 MB. However the network adapter of the iMac should auto-sense the slower speed and adapt to it. I know my wife's PowerBook auto-senses and works just fine with 10 MB ethernet devices.
You can use a crossover cable or two normal cables and a hub. Hubs are inexpensive these days and have the advantage of having blinking lights which can help you when troubleshooting the connection.
On both machines use the TCP/IP control panel to set up the configuration. If you need more help, just yell. (email@example.com)
Another good source of info is the Computer-Mac forum on TalkCity.