They'll work together just fine, but bear in mind that they'll only clock to the lowest clocking card unless you use Afterburner etc. A...
No they don't, since 2009 nVidia's Forceware driveres have supported different clock speeds between different manufactured cards. There is no downclocking - for the last 6 years the cards will operate at their independet clock speeds. I have run mixed cards with different clock speeds for years and no issues.
Tomorrow I'll be running Asus GTX 980 Strix and Gigabyte GTX 980 G1 Gaming.