Hello. Simply adding  6x 8 GB memory modules to the remaining slots will trigger non-optimal memory messages during POST and you may be required to press F1/F2 to continue during reboot of the server. The memory channels on both processors need to be balanced and as such you will have to populate 8 GB modules on the slots with white sleeves and 4 GB modules on the remaining slots for optimal memory configuration. I hope this answers your question. 
We have a C6100 with two blades installed, each of which has 6 x 4 GB DIMMs installed (Dell P/N 9J5WF, which are 1333 MHz, 2Rx8 low voltage parts). Is it OK to populate the remaining 6 DIMM slots with 8 GB parts with a similar speed & voltage? Or must I stick with the same 4 GB DIMMs? Andy
Is it OK to mix 4 GB and 8 GB DIMMs in a C6100?