In my limited experience with live bearers (recently anyways), platties are a touch more aggressive or can be. The lyretail mollies I had were more like big cows, you could pick on em but they pretty much just barrel roll ya out of the way and keep looking for food or to mate. I never saw the mollies pick on anything unless it was small and they were thinking of taking a nibble but even then they preferred food given to them or grazing off rocks and the glass.
I had a aggressive female koi betta that chased and picked on everyone but the two mollies. She would go after them but the big female would just kind of smack her with the side of here body and the betta would leave her alone after that (molly was twice the bettas size). That being said it didnt keep the betta from picking on everyone else.
The mollies are a little big for that size tank however.