Combining kits of RAM, even "same" make and model is never a good idea. X79 in particular has not shown itself to be a friend of this. The maximus and that platform are more forgiving but I still wouldn't recommend it. Get a single binned kit for either. For four sticks on the Maximus you could get a single quad channel kit and run it as two dual channel kits....but doing the other way around and on X79 would be asking for trouble if your CPU sample turns out to have a weak IMC (memory controler).
For gaming 16GB is plenty ...of course if you want RAM disks etc. you will want more...
For X79 G.Skill Z series is great or Dominator Platinums
For the Maximus G.Skill Trident X