I response to you questions...
Luke at Tire Rack can give you great advice, but a general rule of thumb....
1) They will slide around in the winter due to an increased contact patch. Larger tires tend to push snow up in front of the tire rather than cut through it.
2) Hydroplaning - ??? only if you put a poor design tire on them.
3) Brakes aren't big enough for them
cough - Bull$****!!! cough
There are lots of people that drive around in the winter on 18 inch rims. They usually have all seasons on them, but I would recommend a true winter tire if you get really nasty weather. It is a compromise to run all seasons.
Why not use you stock rims for winters. The smaller the tire the better it will perform in the snow. Smaller tires cut through the snow and create more pressure on the contact patch for bite in snow and ice.
Just something to think about. If you do run the 18s in the winter, put a GOOD winter tire on them, and drive carefully. And WAX them rims good...don't want to ruin them in the salt.
