Mixing and matching RAM, even of the same model does not always work. This is why RAM is sold in kits as sticks are tested with each other to work. RAM compatibility issues are magnified with Ryzen as it does not play well.
Your best option is to match RAM models. If you cant do that then match RAM speeds and CL. Whatever you do, buy the RAM from a vendor that accepts returns as there is a chance that it wont work at all.