Mixing ECC RAM brands is usually fine.

Yes, as long as the specs match you can mix brands of unbuffered ECC DDR3 or DDR4 without issues.

The key things to match are speed (e.g., 1333MHz or 1600MHz), voltage (1.35V vs 1.5V), and rank (single vs dual rank). Mixing different speeds will just force everything to the slower stick’s speed. Different voltages can cause instability. Rank mismatches usually work but might lose dual-channel mode.

Used ECC sticks are a great deal. Just run a memtest for a few hours before trusting them with your Proxmox VMs. A single bad bit can corrupt ZFS.

Don’t overthink it. Grab a matching pair if you can, but brand alone won’t cause problems.