well, they have to be both compatible with your computer, other than that it usually works if you mix brands of ram and speed of ram. However if you do mix them they would run at the slower one's speed.
mixing memory is not good. not only do u have to consider speeds but u also have to consider the type of memory. is it 184-pin or is it 240 pin also i an bet that older ram and newer ram will not fit let alone be compatible.