Both would work (18x8.5 33 and 38) However if you can choose, I would go for 33 as its less tight on the inner struits than 38.
You could run a 18x9.5 square setup with +35 or even +38, however it is super tight, and also pokes alot in the front. I would not advice it, but it works. Check out this thread for example
http://www.ft86club.com/forums/showthread.php?t=11222
You're welcome, I've spent alot of time with this when choosing my setup and had about the same questions, so why not pass on the stuff I learned to my fellow Twin-owners.
18x8 is perfect for 225, and on a 18x9 I would go for 245. Those are also more common sizes and shoud be cheaper than a 255 for example. If you need help with tire flank size just let me know.
Square is pretty much possible in all sizes, I myself have a 18x8.75 +35 with 245 tires all around, with 1" drop on springs, I would not call it a skinny setup.
Difference from Square to staggered, appart from the obvious convenience things like being able to use the same tires front and rear and swap them if you need, depends a bit on your overall setup, and what offset you use. Generally, if you have a wider ...gauge... (not sure if that is the proper english word. That wide between the wheels) the car will be more stable at higher speeds, but looses agility in cornering. But overall, you dont really notice the difference that much unless driving on the limit