Per the Fair Use Act, as long as you don't use it for profit, you're fine. Otherwise, you got to contact Lucas Film for permission to use a world in the Star Wars Universe (even if it's just in the books).
Generally speaking yes, but I would be careful regardless - make mention in a blurb before you post that your are aware you're making use of another author's concept so there is no confusion/backlash. I think posting even if on the internet (without making a profit) is still considered plagarism to most, and is frowned upon in most cases.
Would it really be so difficult for you to just create your own world? We are writers after all, it's what we do I just think, to save yourself the headache, it would be worth the extra hours brainstorming.
If it's something like fanfiction, then you don't really need to worry about anything. If you're intending to publish or use it in a portfolio or non-fanfiction affiliated contest for cash or credit, then be careful. Overall, I think that writers are generally pretty forgiving, as long as the work is just for practice or fun.
One guy made a website about a lot of the magical creatures in Harry Potter and J. K. Rowling sued him, so if you're planning to do something like that then be careful
I just think, to save yourself the headache, it would be worth the extra hours brainstorming.
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