Should I mix ECC and non-ECC RAM?

Probably not. Many boards will boot but disable ECC, treating both sticks as plain DDR. Mixing is “not recommended” because the memory controller wasn’t validated for it—you risk random crashes, boot loops, or a system that only works with one stick. If ECC matters, use all ECC sticks; if not, buy regular RAM. Mixing almost never saves money or improves reliability.