Re rats v rabbits as food... you might as well throw cows, chickens monkeys or sheep into the equation also. It is true to say that for reptiles, meat is meat and rat meat has near enough to the same nutritional value as rabbit meat. The significant point is that meat is not fat, and in this regard, reptiles are not dissimilar to people in that too much fat is bad, particularly in conjunction with very little exercise.
I was recently reading a thesis on the biology and physiology of snakes, which stated that the author raised a clutch of carpets, half on beef steak and half on the usual diet in order to research the issue, with no detrimental effect and no difference batween the two groups.
The rats v rabbits debate normally raises it's head when larger feed items are required. My observation is that once rats reach over 300gms they become fat. This being the case, if you want to feed your python 500gms of food you are better off with a 500gm rabbit or 2x250gm rats rather than a 500gm rat which is more than likely overweight. A more easily available alternative may be half a chook or some beef shank. The food value is much the same.
Perhaps someone can tell me if skin and hair are important?