If you use a Mac you can't boot from an USB, you have to boot from a CD.
Sure you can. I have a late 2013 macbook pro retina, double boot set up with OSX and linux, and I boot to USB often for various tasks including paper wallet creation.
Just use rEFIND, the boot menu tool for EFI based machines (forked from rEFI after that project stopped development):
http://www.rodsbooks.com/refind/