New to cider making
 

I just started my first cider which is from a pasteurized variety from the grocery store. It did contain potassium sorbate but I made a gallon of juiced apples and used it as a starter. After adding the starter I then added yeast nutrient to my five gallon batch (1 tsp per gallon). It started fermenting fine after about a day and for about a week now. I am wondering if I already messed it up by adding the yeast nutrient all at once or should I just leave it and see what happens?

I think you're going to be fine. Using pasteurized juice with potassium sorbate would have initially had me saying "no no, that wont work"... LOL :) I'm glad it's working out well for you. It may not clear up fully when it's done because of the pasteurized juice, but it would still be completely drinkable- just not as showy.

Let us know how it turns out in a few weeks/months!

So my cider has been in the primary for about two weeks and my FG is around 1.020 (OG @1.050; abv: ~3.0%) and fermentation has slowed to about 20 sec. per bubble in the airlock. Should I let it go for another week or should I rack it to the secondary? I tasted when I took the readings and it tastes great (i.e. it hasn't turned to vinegar so I must be doing something right.).

I would leave it for around 3 weeks total time in the fermenter.

OK so I'm going to leave it for another week. I plan on racking it to my 6 gallon glass carboy. Should I NOT leave a head space? Also after its been in the secondary for a week what should I do to pasteurize it? Since it was already pasteurized (with potassium sorbate) do I even need to? It seems to me like the yeast is not dividing due to the sorbate so I feel like I don't need to pasteurize it further. Also I am worried about bottle bombs, I plan on adding about a half cup of dextrose for carbonation but I'm unsure about whether they will explode or not.