Can I mix RAM brands and speeds?

Yes, physically it will work, but both sticks will run at the slower stick’s speed and timings (e.g., 2400MHz if one is 2400MHz and the other is 3200MHz). Stability is the bigger issue—different brands or batches can cause crashes or boot failures. For a new build, buy a matched kit to avoid headaches; if you already have one stick, mixing is worth a try but you’re rolling the dice on performance and reliability.