The 3X Pro can fit even a Red Epic so a 5D/7D/D800 is no problem.
The Zenmuse is 1.3kg The 3X Pro with mounting adapter is 1.1kg. A Canon 5D with 28mm prime is about 1.25kg
I just weighed the S800 with 3X Pro v2 mounted, two 6s4000 batteries, and simulated ballast (camera) weight. Total comes to 6.4kg. Just over the published max by .4kg. Will be interesting to see how the copter performs.