If I were you, I would stay away from yoplait altogether. It's loaded with lots of artificial stuff. It looks nice, and they boast about it's weight loss benefits, but it's not all it's cracked up to be. It is true, fat free is different than skim. Skim may have traces of fat, but keep in mind, not all fat is bad and not all saturated fat is bad either.
Yoplait uses lots of stabilizers and such to give the fat free a creamier texture. I'm not sure if I'd want to eat all of that stuff. Definitely don't go for the sweetened yoplait, they use high fructose corn syrup.
I switched from yoplait to Trader Joe's yogurt. I eat the cream top kind, but if fat is a concern for you then get the skim. If you don't have a Trader Joe's in your area, then look for any natural/organic yogurt that is unsweetened or fat free.
Yes, yoplait comes in a pretty container and it comes with that little "best life" stamp of approval. But there are better options.
And, you're not going to find any true sugar free yogurt that has no grams of sugar. Keep in mind, lactose is a sugar too. You will only find unsweetened.