I gave my best friend a bridal shower luncheon at my house and took great pains at making sure everything looked really nice. I bought a new tablecloth and put my floral placemats on top. I didn't have enough casual china for all 10 guests to have a matching set, so I mixed the 2 patterns that I had. My best friend's soon to be sister-in-law told me, in a loud voice so all could hear, that I wasn't supposed to put placemats and tablecloths together. She also stated that the 2 patterns didn't really go together. My best friend tried to smooth things over by saying she thought everything looked really pretty together. Did I break an etiquette rule with putting placemats on the tablecloth?
You did not break any etiquette rules, but the sister-in-law did. She was out of line to give you unsolicited advice, especially in front of the other guests.
Normally, one uses either a tablecloth or the placemats, but not both. But, it's not a big deal if tablecloths and placemats are put together, and not considered a breach of etiquette to do so. So, combine them the way you like.
Also, it's fine to mix patterns of plates. It's fun and interesting to combine styles.
It's too bad the sister-in-law has such a rude nature. Feel sorry for your best friend who has her for a sister-in-law.