To answer a few of your questions:
1. My '00 XJ's door jamb sticker states the stock tires are 215-75-15, but the Jeep came from the factory with the optional 225-75-15.
2. You can mount either tire on your rims (or buy dedicated rims (Soft-8s) for the winter tires>>that way you can change over spring-fall for free!!).
3. You won't have any problems switching tires. Smaller tires will make your speedo read "faster" and will numerically increase your axle ratio-better for towing, etc. In the winter, I run 225-75-16s snow tires ("borrowed" from my GMC full-size pick-up) on Soft-8s and the speedo is dead nuts compared to various GPS measurements.
4. Skinner tires are better for snow conditions, as stated above.
5. As far as snow tire selection, I am running Dean Wintercat SST studded snow tires (I snowplow with my XJ). They have awsome traction (even probably without studs). Dean tires are made by Cooper, but cost less. The Wintercats are a knock off of Cooper Discoverer M+S, which are highly rated in the snowplow industry. I used to run Dean Wintermaster Plus, (50,000 miles winter & SUMMER use), but they are no longer made in 225-75-16 size. Although I bought them local, check this link: http://www.vulcantire.com/privtire.htm