Buy a matched kit.

Yes, you can mix RAM from different brands, but it’s a gamble. If you already have one stick and want to add another, it might work, or it might cause crashes, bluescreens, or just run at slower speeds. Matched kits are tested to work together, so that’s the safe bet.

The biggest risk is timings and voltages. Even if two sticks have the same speed rating, their internal timings can differ. Your motherboard might automatically run both at the slowest common denominator, or it might just refuse to boot. You’ll spend more time troubleshooting than you would have spent buying the kit.

RAM prices are cheap enough that the savings from mixing are tiny. A 16GB kit of decent DDR4 costs like $30–40. Is that worth random reboots? I don’t think so.

If you’re on a strict budget and already own one stick, try it. If you’re buying from scratch, just get the kit.