Mixing RAM modules is a 50/50 gamble. RAM modules are only guaranteed compatibility in the form sold (from the exact same pack). Even modules of the exact same model are not guaranteed.
The manufacturing process for each RAM module changes frequently, so I could actually buy a particular model RAM module one day, then buy the exact same model the next month, and they will not work nicely with one another, as despite being the same model, they have been made differently. This is why RAM is only guaranteed in the form sold (from the same pack). Not saying it never works, but it's a gamble.
Best practice and what I personally always recommend, is avoiding the potential headache by buying a whole new pack. As the last thing you want to do is spend money to find out they don't play nicely together.