R9 280 and 7950 can be crossfired, the 280 is basically a rebadged and slightly overclocked 7950. You can mix vendors (i.e. XFX with a ASUS), as long as they are the same GPU.
As long as the base model is the same you can technically crossfire different after-market brands, though it is often recommended to use the exact same make/ model if you can.
R9 280 and 7950 can be crossfired, the 280 is basically a rebadged and slightly overclocked 7950. You can mix vendors (i.e. XFX with a ASUS), as long as they are the same GPU.