I have one rat at the moment. I used to have two, but one had to be put down due to a very large tumor in her stomach. The first one, the one who was put down, was noticeably different in the way she interacted with people and objects after getting the new baby girl. Being blind, she was a bit fearful, but after getting the second rat she changed completely. She explored a lot more and loved to cuddle and groom with Nellie, my current girl. She also became friendlier to humans (she was friendly before, but shier.) So there is a significant psychological difference between keeping a pair and keeping a lone rat, due to their high social requirements. A human cannot groom and cuddle and sleep with a rat 24/7, so I have never recommend single rats to anybody.
Once I move in October, I will be looking into getting a new friend for her, since she has been sad and lonely after losing Bella.
For food, mine's the best I can find at the LPS, and its called Reggie Rat. I notice that she ignores the corn, but I think thats a good thing?
For a cage, she has this one:
It is large enough to hold a trio of females or a pair of males.