Draser mate, don't rip stuff from other games and intellectual properties to put in your map. You'd do much better if you made a map based on the layout of the original without using any Star Wars content.
~stuff~
As long as due credit is given, there's nothing wrong with modifying any existing content.
No, no and no, again.
Just because someone has been credited does NOT mean you have their permission, and hence does NOT make it OK to use something they've created and published. Publishing your work doesn't mean that you give up all and any rights to the work you've made. Therefore just because it's published doesn't mean you're free to re-release it or modify it.
In this particular instance we're talking about content from a commercially released videogame which is undoubtedly protected by copyright law, meaning that copying or modifying it's content without express permission is not only unethical but illegal.
The only way that it would be OK to republish someone else's work is if you obtained express permission from the original author and publisher before hand.
I'll give you an example. Bob goes to work and creates a great model of a car. It's a car he's loved for years and has been waiting to put it into a game for a while. The game finally gets published months later and he's really happy to see that his car made the cut. After work the next day he goes home and fires up TF2. He joins a custom server and starts playing. "What the shit!" he exclaims when he see the car that he spent a week modelling in a map released by "1337M4pp3R". Nowhere in the map does it say it's his model, nowhere in the in-game info either. Somewhere buried deep in a forum somewhere on the net there may be a post that states this model was made by him but no-one will ever see it. The next day he goes to work and contacts the companies legal department.
Alternatively, how would you feel if someone decompiled your maps and used portions of your designs without asking you first? Imagine their map went official because of the work that you put in, would you not be annoyed?