The only time I know of was (I believe) the 5770 and the 6770 could be crossfired together. This was ONLY because they had the same core. The 6770 was a rebranded 5770.
So say, On Nvidia side, If you wanted to SLI (Crossfire) cards, a GK104 chip can only go with a GK104 chip. Which Is usually always the same card.
6870s can only go with 6870s. 580s can only go with 580s, etc.
Unless of course they have the same chips. This is what I've Always read anyway