Can I mix different brands of ECC RAM?

Yes, as long as the specs match — speed, voltage, and rank — you can mix brands of unbuffered ECC DDR3 or DDR4 without issues. Mismatched speeds force everything to the slower stick, different voltages can cause instability, and rank mismatches might lose dual-channel mode. Used ECC sticks are a great deal, but run a memtest for a few hours before trusting them.