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AFAIK you can use it and trying it won't break anything.

EDIT: Is it desktop or laptop RAM (DIMM or SO-DIMM?) iMacs use SO-DIMMs (laptop RAM) so if that's desktop RAM it won't fit
 
Nope. First, because you need SODIMM, not DIMM for your iMac, second cause it's chipset is not compatible with ECC memory.
 
Good point about the laptop RAM, I didn't even think about that.
 
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