This set is using 2 types of frames, different colors, and colors. I like how they have the same mat in each one to give it a common thread.
This one is in my hallway going upstairs. Another grouping using different types of frames.
When I worked at Pottery Barn, a lot of my customers wanted to create a photo wall, and asked my opinion about if they should use the same or different frames on their wall. I told them it's about personal choice, what kind of feel you want your walls to have.
I call it decorating "suggestions" not decorating "rules" Do what you want and what looks best to you.