Unganged and mixing two RAM pairs – different size & latency

I have Gigabyte 880GM USB3 board that supports 4 memory slots.
Dual Channel slots are 1-3 & 2-4.

I have Corsair 2 + 2 GB (1333 Mhz CL 9) RAM sticks in slots 1 & 3. Now I wish to add Corsair 4 + 4 GB (1600 CL 11) in slot 3 & 4.

Motherboard manual mentions

When enabling Dual Channel mode with two or four memory modules, it is recommended that memory of the same capacity, brand, speed, and chips be used and installed in the same colored DDR3 sockets for optimum performance.

However can I safely mix pairs? E.g. Two 2GB identical in one channel and another two 4GB identical but different from first pair in second channel.

Also if I set it as Unganged will two pairs operate independently with their own latency or regardless of Ganged or Unganged Mode the entire 4 stick combination shall reduce to the lower denomination? E.g higher latency 11 and lower speed 1333 in this case?

Thanks.

It will operate at the lowest speed. Also, i doubt whether it will work in dial channel mode.