How can I downgrade a mid-2010 21.5" iMac from Yosemite to Snow Leopard?
Your iMac was delivered with 10.6.6 (somewhat later with 10.6.7).
When you have the original (grey) discs that came with it you can use disc 1 to install the Snow Leopard version (not the disc of another machine). Then update it to 10.6.8