+1 to what Insomniac said.
Juice often has preservatives in it. The only way to be sure is to check the ingredients list for words or compounds containing 'sulfa'.
Also, acid makes life harder for your yeast. I always add it to taste after fermentation for that reason and also because the cider taste will change significantly. If you have access to a juicer it's better imo to up the acid with tart apple juice (Northern Spy & Granny Smith are two of my favorites) then with an acid blend.
The amount of metasulfite varies depending on the brand of tablet. One campden tablet is not enough to inoculate 5 gallons. The container should list the PPM per gallon of a single tablet. Use this to calculate how much you need to add.
In all the states no door stands wider,
To ask you in to drink hard cider