Fleas, Food allergies, or Dermatitis?

Question
Earlier this year I adopted a puppy from the ASPCA. She had ringworm that was being treated and I continued the treatment when I brought her home successfully and for a while she was fine and healthy. That was when she was about 3 months old. When she was about 6 months old I noticed she was scratching a lot despite having a bath once a week every week and not being around any dogs with fleas. I changed her food to one with no corn wheat or soy, 18% protein, expensive. It helped a little bit, but she was still scratching more than I was comfortable with. I hit some hard times and moved in with a roommate who has two dogs but hardly ever bathes them(actually I end up bathing them out of pity, but I can't do that as frequently as needed)and never does anything about the fleas. Of course, my dog has fleas now, as well as a cat I'm watching for my sister. The fleas don't seem to be the entire problem though, since when she scratches and bites, I can't find the fleas in that area and her skin gets scaly and inflamed with patchy hair loss. I have never seen this on a dog with just fleas, and so few of them since I bathe her often, pick fleas off her whenever I see them, and limit her time around the other dogs and their areas. I've been using a local over-the-counter  food supplement containing vinegar, garlic, and brewers yeast to prevent fleas which has worked well until I moved here, and am hesitant to try things with pesticides and poisons in them, however slow-acting they may be. Can you recommend anything that is easy on dogs with sensitive skin, safe, bath-resistant, and could take on a large-scale infestation like this? I can't afford to medicate all the dogs, only mine and the cat, and plan on moving soon, but I need to do something now and can't afford to go to the vet until my hours pick up at work. I don't know whether she has a skin allergy to the fleas or a skin allergy to something else or just is prone to rash-y skin, but the fleas are a big problem. I think I included all the really pertinent information here... Where do I start to fix this?

Answer
There are not many natural remedies for fleas.  Most shampoos will kill fleas, but that has to be used every day, which is not good for a dog.  Probably the safest non-toxic flea medications to use are selamectin, in REVOLUTION of fipronil, in Frontline.   Many other products work better, but they contain some more toxic pesticides.