I wouldn't worry about gutloading them separately then. But I would add a little bit of multivitamin to the dusting mix once a week, and you should be fine. And I would just use that indoor formula you mentioned for all of their calcium, especially the possibly gravid female.
As for how many, if they are 2" supers or so, anywhere from 1-4. Crickets will probably be about the same, maybe a few more depending on the size. Hard to say for sure, especially since they came from petco, who are not renowned for their care and feeding of animals. Once you have them established, and eating well, they may skip a day, that's normal. Most people with healthy adults feed them every 2-3 days. I aim at every other day, except with my breeding females. I feed them whenever they will eat. They will go off of food for a few days before laying, sometimes as many as 4 or 5, sometimes only 1 or 2.
