I've never tried mixing light and dark brown sugar together. The way I look at any recipe is that if it calls specifically for light brown sugar then I use light brown sugar. If it calls for dark brown sugar, I use dark brown sugar, and if it just simply says brown sugar, I always use dark brown sugar. So, for chocolate chip cookies I've always only used dark brown sugar. To get them to rise more I refrigerate the cookie dough and bake right out of the fridge. I get lots of family and friend requests for my chocolate chip cookies and I only use the tollhouse recipe on the bag. I think one other thing I do that helps with them is that I only mix my ingredients by hand, never use a mixer. I think a mixer breaks up the butter too much, but I'm not sure why it's better only mixing by hand.
I somehow don't think I actually answered your question, but if my experience helps at all, then I guess it was ok.
I'm not really sure it would matter. I believe the only difference is that dark brown sugar has more molasses in it so you may get a stronger tast of molasses in your cookie and they'll be darker.
That is correct. You can use a mixture or even all dark brown but the molasses flavor will be stronger. 
measurement is very important, and you neeed to pack the bs tight in measuring cup...I always use dark brown sugar in my ccc
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