ram speeds at XMP/DOCP profiles are set to that particular ram model/matched set
ie a single stick rated at 3200mhz is rated to run 3200mhz by itself, doesn't mean that it will defintely 100% of the time run 3200mhz when paired with other ram. this also applies to pairs of ram - if you buy a matched pair, they're guaranteed to work together at that rated speed - but might not work at that speed when that pair is working with another set of ram.
all ddr4 ram above 2400mhz is (as you say, overclocked) but they'll work at 2400mhz, which is the jedec standard speed. so if you have any incompatibility with non-matching ram - then they all should run when you set it to 2400mhz.
Conflicting answers. So let me add more detail.
to answer in your case specifically. given that ryzen/threadripper is more ram sensitive, it may make more sense to get a quad channel set.
but yes, as myself, and armageus have said, it's mostly okay, most of the time, to mix-and-match ram.