Small File/Printer Sharing Network

In our office we currently have three useable Macs, one G5 running OS-X 10.3.9, and two G4's running OS-X 10.2.8. We are also considering adding two new G5's. We'd like to network them all so we can share files and a printer. However, each is connected to our school's network seperately for internet access, and Computer Services does not permit the use of routers (They also know nothing about Macs). Does anyone know a way we can set up a file/printer sharing network just in our office while still having each mac with its own connection to the wider network for internet access? (The G4's both have regular Airports, the G5 doesn't)

Put a second NIC Card in the G5 and use a switch instead of a router.

Internet into G5 NIC (1)
Use internet sharing from G5 on NIC (2)
Ethernet out of NIC (2) to switch
All other Macs to switch to get internet and Peer to Peer sharing

Technically the G5 is acting as a router, but that would suit your needs. I assume the no router rule is to monitor all nodes that access the internet. All the Macs internet traffic would show as coming from the G5. Other wise you'd have to put a second NIC in all the Macs and connect the second nics to the switch and use rendezvous for sharing.