1) 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.
It is better to wait and get what you need in one kit. Then sell the old ram to recoup the cost.
2) With an odd stick, some ram, if not all will operate in single channel mode. I think this hurts cpu performance on amd more than intel, so be advised.