Is Flouride harmful to house plants?

No, it’s not something they need, but it’s not harmful.

Nope.

I would worry more about chlorine. I notice the difference when I water my plants with rain water versus tap water.

I found this.
Apparently, some plants are more vulnerable to fluoride than others. It explains why my spider plant which is fed mostly tap water gets miserable during the winter months. In the summer they do way better outdoors and some inside will be fed mostly rainwater in the summer and spring months and they get full and lush.

No.

There is usually a lot more chlorine than fluoride in tap water, so worry about the biggest effects first.