Can I mix 1.5V non-ECC RAM with 1.35V ECC RAM?

Probably not. Even if you manually set the voltage to 1.35V, the 1.5V non-ECC sticks might not be stable or compatible at that lower voltage. Your motherboard requires ECC memory, and non-ECC sticks won’t work even if the physical slot fits. Most motherboards that demand ECC memory simply won’t boot with non-ECC installed, regardless of voltage. Save yourself the headache and buy the correct 1.35V ECC DDR3L sticks.