While it's never a good idea to mix DRAM if you have to, best to go with the same exact model (and even that can be problematic - that's why they sell sticks in a variety of amounts and the bigger sets cost a little more - takes time to find 4 sticks that all play nice together
) but with same exact model can often get them to play with minor voltage/timing adjustments if they don't want to play out of the package