AMD already confirmed all R7-R9 cards would be compatible with their equivalent HD7000 models. R9 270X is a rebadged 7970GHZE not a 7850. R7 260X is a rebadged 7790. As far as I know, there is no equivalent to the 7950 or 7850 as of yet. A R9 280 should come out which logically should be a rebadged 7950. There's also R9 270 that is also rumored so I'm guessing this one will be a rebadged 7850. That's the one you'd be looking at.
They successfully CFed an HD7870 and 270x. My card can be crossfired with the HD7870. Is it not safe to assume that it will also be crossfireable () with the 270x?
It's definitely not safe to assume that as you may run into issue. It's something that is probably feasible but having owned neither cards nor having tried to CFX two different models, I can't say for sure.
Edit: Also, it would be a bit silly to CFX a 7850 with a 270X even if you could. The 270X will downclock itself to run at the same speed as the weaker 7850. You'd pretty much get the same performance as two 7850 at best. It's pretty much like buying a 2nd 7850 for the price of a 270X.