I'm considering buying Snow Leopard Server, however, the person I'm buying it from will be providing me with the server discs that came with his Mac Mini Server and a serial number. If I have an iMac already running Snow Leopard, will I still be able to take the server discs included with the Mini and install server on top? Are the discs compatible with the iMac? Thanks.
Unfortunately, the disks that come with a Mac are Mac-specfic. They have special drivers for the machine they came with, and usually don't include drivers for other machines. The serial number he's providing you with should still work with no issues, but the disks themselves will probably not work. I would suggest getting the Snow Leopard Server disks from another source and using the serial number you're buying.
As for installing Server on top of the existing OS, this shouldn't be an issue. I would still recommend having a backup, but it shouldn't erase any data.