I use a Mac Pro with Lion installed and I own a broken MacBook Pro with Snow Leopard on it. Am I allowed to install Snow Leopard on the Mac Pro in parallel to Lion? The MacBook Pro is totally dead and useless.
If it is a retail copy of Snow Leopard, then yes. If it is a machine copy, then probably not.
It's unlikely you'll have any problems installing it from a licensing perspective, there is no serialisation in Snow Leopard. You may run into strange problems down to the incorrect build of 10.6 for the hardware though as discs that come with machines are machine specific.
Snow Leopard is cheap though, you should buy it if you think you need to!