Whoa, lots of questions! But all good : )
I'll hit some of them now and then come back to them later.
Carpet: when you arrive you can buy cheap carpet in the library parking lot I believe... this is probably the best way to get the right sized carpet for a dorm room. However, I bought a carpet before I came to school that just filled up the middle of the room (probably 6x8 ft or so? I have no idea what typical carpet dimensions are) and that worked fine too. I just liked having it because the floors are tile and might be cold on your feet during the winter if you didn't have carpet.
Bunkbeds & couches: Bunking your beds WILL give you a lot more room, which is cool. I did not have bunked beds in my room, because I figured if my bed was on the bottom it would become like the couch, and I didn't really want people sitting on it, and I figured if it was on the top I couldn't study up there. However, in my sorority house, I now live in a room with no beds and a couch and I like this much better. SO, my point is, I recommend bunking your beds but having a couch too so that the bottom person's bed does not become that. The reason being that studying on your bed is a bad idea because you will probably fall asleep while doing your homework, AND if you only use your bed for sleeping (like I do now) then as soon as you lay down in it your body knows what's up and goes to sleep a lot faster. Having a couch might also make you the cool room to hang out in since people could easily gather there to watch a movie. I thinkt this would be fun, but if you're not a very social person or just like your space without people in it, this could be a disadvantage to bunkbeds and couches.
Closet tips: put your dresser in your closet - it will give you a lot more space in your room and the closets are so big (at least in Schwitzer) that they still hold plenty. I have heard this may not be true for Ross, so maybe someone else can comment on that. Also, if you like to hang a lot of clothes like me, buy a second rod thing (sold at places like the Container Store, Bed Bath & Beyond, etc.) This hangs down from your closet rack and creates a second rod, giving you twice the hanging space. Sweet!
What clothing to bring: If you're going home at all during first semester, don't bring all the clothes you need for the entire year. For example, it never gets too cold before Fall Break, so if you are heading home then, don't bring all your sweaters and winter stuff until after break. A few hoodies and a jacket can tide you over.
Butler's Drainage system? invest in rainboots. I am pretty sure that most student's biggest complaint about BU is the drainage system. When it rains, there will be a lot of water on the sidewalks in some places, so buy some rainboots or something so you don't have to sit in class with soaking jeans! The cheaper version of this that I use is rolling my pants up and wearing flip flops, however, this is painfully cold in the winter lol.
Schwitzer & Ross in August = Bring a fan!!! There is not air conditioning in these dorms, and the first few weeks will be much more pleasant if you have a fan or two.
Bring your school supplies. I was so focused on moving to college and dorm life and whatnot that I totally forgot to bring all my old school supplies (folders, pens, pencils, staplers, etc.) Don't waste your money here buying stuff you probably already have at home.