It isn't against the rules to mix it up. I was just at theonion.com, and they definitely mix it up. The headlines are either serif if the body is sans serif or

It isn't against the rules to mix it up. I was just at theonion.com, and they definitely mix it up. The headlines are either serif if the body is sans serif or sans serif if the body or link is serif. The button navigation is also sans. I think your page would be easier to read if you mixed it up a bit. It just looks like a wall of text which is intimidating. There needs to be differences in the text to distinguish if they have functions or not.