Mixing RAM sizes works, flex mode is fine.

Yes, you can run an 8GB stick and a 16GB stick together for 24GB total. The system will enable “flex mode” (or asynchronous dual channel) – the first 8GB on each stick work in dual channel, and the remaining 8GB on the 16GB stick run in single channel.

Performance impact is minimal for most people. You lose dual-channel bandwidth on that extra 8GB, which matters for memory-intensive tasks like video editing or large compiles, but everyday browsing, office work, and even light gaming won’t notice a difference. The real issue is mismatched speeds or timings – if one stick is slower, the whole system clocks down to that speed.

I’d do it without hesitation if you already have one stick. Just make sure both are the same generation (DDR4 vs DDR5) and voltage. Future You gets more memory now, and dual channel for most of it. Not a bad trade.