You could use Ethernet for all of them, but your System 6 and 7 (7.5.5 can go to 10.4.11, 7.6.1 can hook up to 10.5. 8) machines will NOT connect to anything higher than 10.3.9 unless you use FTP. You're better off using AFP, because you can share disks and volumes (but you can't "share" a "share" unless you use something like Rumpus).
The Ethernet machines will all need a router, depending on how many machines you have, you might need a router or gateway with more than four ports. If you use LocalTalk for the old machines, then a simple daisy-chain all the way up to the bridge machine would be the easiest and cheapest (and the most practical).
The Ethernet machines will all need a router, depending on how many machines you have, you might need a router or gateway with more than four ports. If you use LocalTalk for the old machines, then a simple daisy-chain all the way up to the bridge machine would be the easiest and cheapest (and the most practical).