It won't hurt anything, but don't use 2 different voltages for one thing. There is no teling what kind of weirdness that may cause. It is best not to mix brands if possible as edogwa said, sometimes the different characteristics of the different chips can make things less stable or at worst, cause crashes. Generally, it doesn't though. If you have to get ram with worse timings to match the voltages, that is okay, the system will just go with the slowest timings on all the ram.