If there’s room left on the stick, you can store anything you want.
If I were you, I’d also burn a copy of the rescue CDs onto CD/DVD.
Making bootable USB sticks can be tricky, and you won’t know for sure if your USB stick is bootable in a certain pc until you have tested it in that pc. Some pcs are very picky about USB sticks when it comes to booting, some USB sticks are difficult or impossible to make bootable, some programs are difficult to make bootable on a USB stick, and there are several “standards” for making a USB stick bootable. All this makes it unpredictable as to what will boot on which pc.