I have a pair of Blue Rams sharing a tank with a pair of Rainbowfish, YoYo Loaches, a small school of Harlequins and three Cories.. from what I know of Rams, they can be temperemental and will engage in some fin-nipping, especially around feeding time.
My experience with Black Mollies is that they are the most aggressive of Mollies, so I'm not sure a Blue Ram/ Black Molly combo would work in a tank that size.. bear in mind, my input is based on observation of the two species in separate tanks, not a same tank scenario, so I can't say for sure that combining the two wouldn't work.. I would err on the side of caution though, simply because of what the worstcase scenario could be, and I love my Rams too much to risk them.
I agree with Richard,the black mollie/blue ram mix in a tank that size is going to be a problem . I have a pair of blue rams and they are very territorial my male will chase off my rainbows from his patch and they live in a 46 gallon UK tank .Richard what Rainbows do you have,ive got Red and Boesmani .