I have the SFF and have two slots populated with PC3 12800s so far. I have some 10600s and some 8500s laying around. Can I mix these in with the 12800s?

I got a Xeon on this Precision desktop.

Another question. I have 1 stick of 12800 as well. Can I use this for one slot and leave my other two slots populated? Or do I have to have both extra slots populated so all four slots are used. Otherwise I would have 3 slots populated. Does this matter?