Can I mix ECC and non-ECC DDR5 memory in the same system?

No. Mixing ECC and non-ECC DDR5 modules usually results in a no-boot or random crashes. Most consumer boards will refuse to POST with mismatched modules, and even server boards that accept both will treat the whole system as non-ECC, losing the error correction benefit. DDR5’s on-die ECC is for internal data integrity, not system-level correction. Pick one type and stick with it.