Depending on how heavy your comp gets, I'd say the more RAM the better - 24GB is better than 16GB :-)
Best is to go for 4 banks populated, so 4 x 8GB. If you only use 3 banks, you cannot use the double channel architecture to its best.
that is not a problem
in case of 16gb i will use 2x8gb in addiction to 2x4gb i already have
In the long run you really should have 4 x 8GB forAE
Running 2 different densities for ram requires the boards to run asynchronous timing support. They are not very good at that and this often causes issues since 4GB and 8GB have different timings. I highly suggest you run all the same density either 4GB or 8GB sticks.