Nothing is compulsory as such, when it comes to poetry. Well, perhaps certain usage of words might be, but that's merely my personal preference. I like poetry with words in it. Free verse is okay, using predefined structure is okay as well. I like to rhyme a lot, but even though this isn't my native language, I do the same in my own as well. Might make me boring, but hey... We can all be perfect. I just choose not to be.
I think so. Form is less important in poetry than the feeling it conveys, imo.
Ultimately it's the poet's decision. However, unless said poet has established his poetic prowess in other works, I cannot say that the success of a free verse epic poem is likely at all.