I definitely say to mix them. You do need a unifying factor and that should be the frames. They don't all have to be exactly the same, but they should be in the same "family".
If you go to my hub on creating a photo wall display (http://hubpages.com/hub/wall-decor-photo-display) and take a look at the very first picture (click on the picture to enlarge it) you'll see that all the frames are brown - some light, some dark, and some with gold tones - all in the brown family. You'll also see that there are several color and black and white pictures included in that arrangement. That is my current family photo wall arrangement and I am very pleased with it.
I hope this helped!