This link may help answer some of your questions: http://www.dell.com/downloads/emea/products/pedge/en/installing_upgrading_ddr3_memory_11g.pdf
You must use the same size and rank in the same channel. When you mix and match, you will get a warning that the configuration is not optimal. Then you have to manually hit a key to continue each time the server restarts. So for best practice, use the same DIMM's in the channel.
Regards,