I agree with your assestment, the art in this picture is just amazing and Taymor should have been nominated for Best Director. During the same time the movie was released I was part my high school's take on the subject, and we had the fortune to feature the actress that plays her mother in the film (she's well respected in Mexico) in our production as Frida narrating her life from her studio.
I actually read the biography the movie was mostly based from, it is a very revealing and comprehensive book, featuring parts of Kahlo's personal diary and even some correspondence with her friends, so you could actually get to know her take on herself and her art.
Rivera has been always been considered one of the best exponents of Mexican mural painting movement. Frida Kahlo, on the other hand, was mostly ignored until the late 80's, as the press mostly considered her Rivera's wife. However, before Hayek's production, theMexican film Frida, Naturaleza Viva put her on the spotlight for her art and personal story.
As for both of them being Communists, well, they were never persecuted for that, and in fact, they were more "communist" leaning than actual militants. Like most artists all around the world, they wer just vocal about supporting "the lower classes" and the "proletariat", but as every other "intelectual" in my country they were both from the upper middle class, and did very little to actually support any social project.