You cannot SLI cards from different generations. You can't even SLI different models from the same generation. With a few exceptions when Nvidia did a little rebranding. (So no GTX950/960 SLI or GTX970/980 SLI, even though they are the same architecture)
For the most part AMD allows crossfire within the same processor family. So A 7970 and 7950 could crossfire, but not a 7870 as an example.