Motortrend.com is a great website that has been around for many years. They offer relevant reviews on tires and and auto products. I use this web site all the time and it is very good.
No, you don't need to have different ATV tires for different terrains. You can read more at ezinearticles.com/?Different-Styles-of-ATV-Tires&id=3863004 No you do not. ATV tires are universal for all different terrains. They are pretty durable and can handle most anything that you go through.
The different seasonal tires can be very expensive. One can find more information about the different season tires at popular on the web sources such as Tire Buyer and Auto Repair.
You can find out some information on http://www.tirerack.com/tires/types/comp.jsp Tirerack is such a good resources, I used it last year when I was looking for tires.
Bikes styles all have different tires to suit terrain. There are road racing tires, cruising tires, off-road tires and many different wheel sizes too.
I have found that there are a few sites available that will allow you to read reviews for tires. One site that I found very helpful with numerous reviews on sixteen different tire brands is the tire rack website.
Depends what tires they are because different tires have different densities...
First off you will have to install different wheels. Assuming you have 16" wheels to install the tires on the answer is yes. This is an very acceptable substitute. Your speedometer will read 60.3 at a true 60 mph with the 16" tires and your engine will turn 4.2 more revolutions per mile which is negligible. Go for it as you have made a very good choice.
Michelin makes many different tires and they do cost more than other brands, but are well known as excellent tires for grip and mileage.
Snow tires prices varies from brand to brand, ranging from 60 dollars to 200 dollars per tire. They are more expensive than regular tires. They do have more grips than regular tires, because they have a different rubber composition.
If the snow tires have a bigger diameter than the others, the odo and the speedo will read a bit low. If the snow tires have a smaller diameter the odo and speedo will read a bit high.
Continental tires recommend different tires for different occasions. The weather affects what tires that should be used, as does the land that is traveled on.
You can read reviews for the new Goodyear Eagle GT tires at quality-reviews.com. Here's a link to a review of said tires- http://quality-reviews.com/my-review-of-the-new-goodyear-eagle-gt-tires.