Don't mix ECC and non-ECC RAM.

It will either not work at all or cause instability.

Consumer motherboards from Intel and most AMD chipsets don’t officially support ECC RAM. Even on boards that kind of work with ECC (like some Ryzen setups), they just ignore the ECC feature and treat it as regular memory. If you mix sticks, the system will either fail to boot, run at reduced speeds, or give you random crashes. You’re not getting the error-correction benefit anyway on a consumer board, so you’re just adding a headache.

Just buy a matching pair of non-ECC sticks. It’s cheaper, it’s simpler, and it’ll actually work.