Correct and yes you can use the same ISO for all blades (I’ve done it before).
I’m just trying to get it to boot into the OS install without having to select the system from the cobbler menu.
I would recommend the following: Use "cobbler buildiso" to create the ISO as normal, however once it's done go into the working directory and edit the isolinux.cfg file to change the default boot option to the entry you want, and then run the mkisofs again manually (the full syntax of the command should be shown in /var/log/cobbler/cobbler.log, if not I can give it here). This should get you what you need.