I did this. I bought a PNY card from BB. They stopped carrying it so I got the same from EVGA for sli. The PNY isn't reference design, but both run the stock clocks for the model. It looked kinda wonky having 2 cards in sli with different lengths as well as brands. It didn't create any issues for me. One of the cards overclocks a whole lot better than the other so that's really the only thing I missed out on, by having them both run at the same slower speed.
I didn't have any gap between the two cards, so I put the one that on its own ran the coolest in the top slot, as it would normally run 10-12 degrees warmer, but still well within the limits.
You should be fine.