I would have to presume the Apple store person knew what they were looking at, but they may have slipped up. The 8,1 designation is for four very different Mac models. Newest to oldest:
1) Late 2011 MacBook Pro. Shipped with Lion, 10.7.x. With very few exceptions, you cannot install an OS older than what a Mac shipped with. So no matter how you try, you cannot install Snow Leopard on this model.
2) Early 2011 MacBook Pro. Shipped with Snow Leopard 10.6.6. If you got the gray disks with the Mac (which you should have), then you can install SL from there. A retail Snow Leopard disk will not work. The newest version retail disk was 10.6.3 and does not have the hardware drivers on it for that model line. You would first have to obtain replacement disks from Apple.
3) Early 2008 iMac Core 2 Duo. Shipped with Leopard, 10.5.x. A retail SL disk would work on that, and just qualifies for Mountain Lion. But since your Snow Leopard disk won't work, the one you have would likely be 1 or 2.
4) 2004 iMac G5. Will not work at all. A G5 is a PowerPC CPU. Snow Leopard and newer are for Intel based Macs only.