New iMac with El Capitan won't start Yosemite usb installer

You may need to use Disk Utility to erase the hard disk or create a separation partition to install 10.10 on it. Note, the latest iMac was released October 16, 2014. This means if the installer key was created from the available downloads before November 17, 2014, then it is too old to be used on the iMac. The only exception is if you created it directly from the restore partition when you bought the Mac. You might be able to consult with AppleCare to make it available for download again on your restore. It might also restore using a command-R boot using no key at all. Regardless, it is unlikely you'll be able to install 10.10 directly over 10.11. You'll need to migrate the files over that are compatible with both once the installation is complete from a different drive or partition.

When was the Yosemite installer downloaded? When Apple releases a new computer it often requires drivers (special code) that exists in newer versions of the OS installer but not older ones. For example, I own a MacBook that will run Snow Leopard but not just any Snow Leopard installer will work - it has to be the 10.6.8 installer. Any previous version won’t boot the computer. If your installer came out before the new iMac was released it is very possible that it cannot be used on the new computer.