Luck.
Seriously, that's what it's probably going to take because there is no formula for figuring out how to use mixed memory together successfully. It is merely luck, and also how good the motherboard is. Even two different sticks of the exact same memory may not always play nice together. If it is going to work, it will work. If it's not, then it won't.
It would also be helpful to know if that 16GB is two sticks of 8GB, or one stick of 16GB, and what the model of your motherboard and CPU are. Not that any of that will change the outcome.