[FionaVB]

1. so good vs really good
These are similar in degree, I think. Both show enthusiasm and a high degree of "good-ness" about the movie. But the word 'really' does not indicate surprise. (The tone of voice and facial expression might show surprise, even if the speaker chooses to use 'really'.)

2. I would say that "incredibly hot" should be hotter than "really hot" but "so hot" comes down to the individual speaker's opinion.
In fact, all of these could show different degrees depending on the speaker's own speech habits. Some people like to exaggerate and put a lot of emotion into their conversation, and use words like 'incredibly' when other people would just say 'quite'.

3. I haven't really thought about 'so' being informal. I guess it is more spoken than written; in writing I think I would be more likely to finish the ellipsis: "It was so hot that three people fainted." 'Very' is a bit boring for spoken situations, and I would normally use other qualifiers in conversation, but I think it depends on personal preference as to when/who/why we would use 'so'.