Kindergarten kids aren't really that cliquey or particular, especially in the beginning.
I agree. I think the commercial is portraying a problem that an older kid would face coming into a new school, and putting a younger kid into the situation to make it more heartwarming. Most kids are still fairly indiscriminately friendly at the K stage, especially in the beginning.
In terms of food, my son wanted PBJ every single day. But since he goes to a nut free school, he had sunflower butter and jelly instead.