Quick question as I sit here at the tire shop...

[darkside4.8]

So my new wheels and tires for summer come on Friday. As luck would have it, I put a non repairable gash in my already worn out rear pirelli (315/35/20) today. Not replacing it as my my new setup is only days away. How bad would it be to use one of my original 18" wheel and tire (255 size) as a spare for the next hundred miles or so on the rear until the new ones arrive? Would I be damaging anything?
Thanks in advance!