All RAM runs at the slowest stick's speed.

Yes, your laptop will run both sticks at the slower speed — 2666MHz in your example. Your 3200MHz soldered RAM gets slowed down to match the replaceable stick.

Doesn’t matter if it’s soldered or socketed: the memory controller picks one speed for all channels. That’s just how DDR works. You might also lose dual-channel mode if the capacities or timings are mismatched, which hurts performance more than the clock speed difference.

Check your laptop’s manual, but usually mixing speeds is fine as long as the system boots. You just leave performance on the table.

If you care about every last frame, match the soldered speed. If you just need more RAM, the penalty is small.