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.