Yes you should be fine, if I read this correctly, you had someone else overclock your computer so that your 1333 ram was effectively running at 1600. If that's the case adding another stick at 1600 or replacing your 1333 with a 1600 shouldn't cause any problems.
However, my comment would be that while swapping out the 1333 for 1600 would ensure your ram doesn't fail due to being overclocked, if your computer is running stable with the 1333 overclocked I would just leave it in. While the lifespan will be decreased, chances are you'll replace the entire computer before your RAM reaches end of life.
So long story short, unless your comp is unstable, I wouldn't fix what isn't broken.