Hi Connie,
I'm glad you discovered the mold before your photos got damaged!
I really love photos going up a stairwell. I especially like your idea of using one staircase for older pics and the other for newer ones.
I think when we have a lot of photos, it is really interesting to make a "gallery" of them for everyone to see. A stairwell, foyer, hallway or one wall in a room is a nice place to do this.
To keep the look more unified, try keeping the frames and background matting the same or very similar color and style. Like black frames and white matting or wooden frames and an antique white/cream matting. This keeps the attention on the individual photos and your eye doesn't get distracted by assorted frames in varying colors and shapes.
Another idea is to keep black & white photos grouped together and group your colored photos elsewhere. The more unified the look, the more the photos will stand out by themselves.
Donna