1. Not harmful.
But ram is sold in kits for a reason.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
That is why ram vendors will not support ram that is not bought in one kit.
Although, I think the problem has lessened with the newer Intel chipsets. Still,
it is safer to get what you need in one kit.
2. To get dual channel operation, you will want the same 4gb capacity on each channel.
Your motherboard manual should tell you how to populate 3 sticks. Some are very specific.
3. Ram speed is not important to modern intel cpu's; more important on amd.
Regardless, you will benefit more from lots of ram than you will from ram speed or timings.
If it is working for you good.
No more needs to be done.