PCPP doesn't appear to allow mismatched video cards for crossfire. I initially bought 2 Gigabyte OC 7970s for Eyefinity before learning that enabling Crossfire disables the outputs on the second card, so I exchanged one for the ASUS DirectCU II shown in the build to fully enable 5x1. The internal pic is from when it was just the 2 Gigabyte cards inside.
When I first started I was super serious about cable management and running everything through the slots and out of the way, but as the build went on I got less interested. No window on the case (not really into windows & lights) so I'm not concerned.
I'll update with benchmarks (assuming PCPP allows me to update this).