Mismatched RAM sticks usually don't run at full dual channel speed.

It depends on the platform, but generally: if the sizes don’t match, you lose pure dual-channel mode. Some laptops (Intel and AMD) support “flex mode” where the matching portion runs dual-channel and the extra runs single-channel. That’s better than pure single-channel but not as good as matching sticks.

Adding a larger SO-DIMM alongside soldered RAM works as long as there’s a free slot, but you’re giving up some memory bandwidth. For most real-world tasks (browsing, Office, even light gaming), you won’t notice the difference. If you’re doing bandwidth-heavy work like video editing or large data processing, matching sizes matters more.

You could also just buy a matched SO-DIMM that equals the soldered size, if the total is enough for your needs. That keeps dual-channel intact. But if you need more total RAM and don’t care about a minor speed dip, the mismatched setup is fine.