Can I mix DDR4 RAM brands?

Yes, usually fine. Two different brands of DDR4 8GB sticks at the same speed (3200MHz) will probably work together, but timings and voltages may differ. Your motherboard will try to match them, often running both at the slower stick’s settings. For default BIOS settings it’s low risk, but enabling XMP increases instability chances. For gaming or heavy workloads, buying the exact same model is safer.