Mac – change the MAC address to be the same as the old MacBook so it will stop rejecting the iPod

The difference you're referring to is the ability to charge over the FireWire pins, which Apple removed from most iPods/iPhones several years ago.

However, as you've discovered, iPod Classics are still able to charge via FireWire. I expect this functionality could be removed at any revision, but for now, consider yourself lucky ;)

This depends a lot on your DHCP server, but I would look into using the client id to assign static IPs. It's not tied to the individual hardware (although you'd have to set it at least once when they first use a TB Display), so it avoids the switching displays problem. It can be set via the networksetup tool.