as long as they are of the same chipset, but the sli will run at the lowest frequency. you could always try them out.
i dont knw if its the same with amd, but they can xfire a 7970 ghz edition with a r9 280x. or a 7870 with a r9 270.
SLI is much more strict in this regard. for AMD card usually as long as both chip have the same chip (the card and gpu core configuration can be different) you can CF them. for SLI even the same card with different name (in case of rebrand) canot be SLIed together.