AMD's/Nvidia's GPUs can only be Crossfired/SLI'd with a exact copy of itself but in some cases with cards from the same gen. such as the HD6950 and the HD6850 (?) is I recall correctly in a case with GPUs from AMD. It shouldn't matter if the card is from a different brand.
The integrated GPU in your CPU (your on-board graphics) can not be in Crossfire with your stand-alone GPU. If it was possible it wouldn't work in your case anyway, as your on-board graphics is two generations over your 6950 according to the numbers.