You don't need to crossfire while mining, and you can mix and match. The settings for each card can be put in separately in cgminer.
No issues with putting in a 280x and a 7950.
Wow, Really? Im just about 1 week old miner, mining with single 7950. So i still dont know how to set up with 2 or more cards.
So you mean just connect 2 cards to mobo and leave it? and cable from the monitor can be connected to any one of them?
And can i control each card separately? like say fan speed, clocks?
You can connect even 6 GPU and control them all.