Overclocking ram is not a guarantee.
Also Ryzen motherboards are far more fussy with ram than Intel ones. So buying 2 more sticks later would typically work on an Intel system so long as they were the same timings.
I am less confident with a Ryzen system based on user comments on here. It might just work perfectly. It might not. Recent updates to the BIOS of most boards have (I am told) improved things.
By default your system will run the ram at their default speeds (be that 2133 or 2400, or whatever). You can select other supported speeds if you have issues. Adding 2 more sticks will reduce your ram command rate from 1t to 2t. This is a fairly trivial drop in performance, but it is a drop.
But, since Ryzen is so fussy about ram, and it benefits so much from faster ram... have you considered just saving up until you can afford 2x8gb 3200?