In the Channel A slots you could have 2 x 4GB, and in Channel B slots you could have 2 x 2GB.
Last caveat: Don't mix CAS latencies. I'm assuming that your 2 x 4GB were bought at the same time from the same manufacturer, and the same is true of the 2GB sticks.
It's OK to have different CAS latencies per channel, but not different CAS latencies on the same channel.
Yes.
In the Channel A slots you could have 2 x 4GB, and in Channel B slots you could have 2 x 2GB.
Last caveat: Don't mix CAS latencies. I'm assuming that your 2 x 4GB were bought at the same time from the same manufacturer, and the same is true of the 2GB sticks.
It's OK to have different CAS latencies per channel, but not different CAS latencies on the same channel.