980 sli with different cards

Hey just a quick question because I don't remember the rules for sli configs.

I have a cheap reference 980 for sale in my area and wanted to know how it works if I put it in sli with my current msi twin frozr card.

Basicly it should be possible to sli them.
However somethimes it can be a pain in the ass with Nvidia cards.

If you can pick it up for cheap, you can allways give it a try.
If it doesnt work, you can allways sell it again right?