Absolutely not. Being too thin is just as unhealthy as being too fat. In those pics you posted the girls look bad and frankly, it looks ugly if your ribs stick out that much. I think to be fit and healthy is most important.
2. Is being over a size 2 make anyone plus-sized?
Of course not! I think a size 2 is UK size 6 and bigger than that is definitely not plus-sized.
3. What do you think about celebs trying to get to the 'perfect size zero'
If they're actresses, for example, their talent is mor important. Yes, they should look nice and obviously exercise, but there is no need to become a size 0.
4. Why do you think skinny girls are considered/wanted to make better models?
Some people might think that clothes look better on skinnier girls. Personally, I think it depends on your figure. Again, in the pictures you posted, the clothes just look baggy. Not one of my friends is a size zero and they all have great figures. I think it's horrible that, for example models can be refused just because they have a waist wider than 24 inches.