Memory timings refer to a set of four numerical parameters named CL, tRCD, tRP, and tRAS, usually represented as a series of four numbers separated with dashes, in that respective order (e.g. 5-5-5-15). However, it's not unusual for tRAS to be omitted, or for a fifth value, the Command Rate (e.g. 1T or 2T), to be added on. These parameters define, in clock cycles, the various forms of latency (responsiveness to random requests) that affect fundamental performance metrics of RAM. Lower (i.e. tight) timings imply better performance. If the equivalent timings can be achieved with a lower voltage even better.
Of the two brands you listed the G.SKILL has the better specs.
No, it is not OK to add the Kingston HyperX. You don't want new memory modules that require a higher voltage than the memory modules you currently have in your system. The Memory voltage setting in the CMOS Setup Utility applies to all 4 DIMM slots. Since the HyperX requires 2.0 Volts and the G.SKILL only requires 1.65 Volts you will be over-volting the G.SKILL modules and probably even damage them.
If you under-volt the HyperX to 1.65 Volts it is rather doubtful that these HyperX modules will even be able to achieve the 9-9-9-27 timings specified.
Why do you want 6gb on a dual channel platform anyways?
I would go for another set of the memory you have for 8gb or just stay with 4gb if you are gaming/everyday usage.
You might want more ram if you are doing some editing or other ram intensive tasks but I would stick with 4gb or go all the way to 8gb.