I'm installing another zone to my force hot water (baseboard) boiler. The initial installation has a shutoff valve and plug on supply and return so extra zones can be installed. On other multizone houses, I've noticed that each extra zone has an automatic air bleed valve on the high point of the supply line, but not on the first zone. My boiler has an air valve on the expansion tank, but not on the existing zone supply. Is this correct? Do I just need the air bleeder valve on the second zone?

Auto vents I'm guessing are what you are talking about. These can be placed almost anywhere, Up high on a loop, with a scrubber in a supply or return, just as long as the piping going to them has a good opportunity to trap any rouge air bubbles. There purpose is really just to help purge some air and make the system a bit more maintenance free. I see hundreds of systems with no auto vent bleeders.