You cannot use an iMac installation disk to install 8.6 onto ANY other machine because the system install disks that come with the iMac (or any system, for that matter) only contain the system files needed for THAT particular hardware.
Only the full version of 8.6 will have all the hardware drivers/files that your 7100 needs.
You should be able to pick up a new copy of 8.5/6 for around $50 or less. Check dealmac.com and ebay and what George Orville said.