It will run at 2666MHz.

Yes, it’ll work, but both sticks will run at the slower speed—2666MHz in your case. That’s how mixed memory kits behave: the system picks the highest common speed both can handle.

The 3200MHz stick will downclock to match the 2666MHz one. No, you can’t force the faster speed. Your motherboard and CPU’s memory controller will negotiate the lowest common denominator. If you have XMP/DOCP enabled, the system might default to 2666MHz JEDEC timing instead of 3200MHz XMP. You’d need to manually set timings to get the 3200MHz stick to run at its rated speed while forcing the 2666MHz stick to match—which it physically can’t.

So you’re stuck at 2666MHz until you replace the slower stick. If you’re after peak performance, running two matching 3200MHz sticks is better than mismatched speeds. But if you just need more capacity and don’t care about a few percent of latency, this works fine.

Just don’t expect any magic upclocking—that’s not how RAM works.