I have a VERY sensitive scalp, even as a child. The red scaly patches you are talking about have been a problem for years. Once I was able to understand contributing factors, I have been able to control it. It runs in the family, too....seborrheic dermatitis.
Sweat- if DD sweats a lot, wash hair frequently. The more I sweat, the more my scalp breaks out. I have to wash mine after every gym workout or on hot days. I have very thick hair and cutting layers into helps my scalp get more air. I also let my hair air dry, but I don't know if that helps one way or the other. Going to bed with a wet head causes breakouts, so I avoid washing hair before bed or I'll use the dryer.
Shampoo- The best shampoo I have found that doesn't cause its own build-up and leaves my scalp feeling clean is Neutrogena T=gel. I also use generics and they are fine. I don't really use it every day, unless my scalp starts getting itchy. I'll rotate it in every few washes. I've used Nizoral, too, but I can get the T-gel in big bottles, which makes it cheaper per ounce.
Shampoos to avoid- Shampoos with fruit extracts (my favorites) really cause a breakout, so I've given them up. It has made a HUGE difference. I can use Pantene OK and Tres Semme regularly with little problem. L'Oreal Fructis & Herbal Essences make my scalp go crazy.
Conditioner- I use a conditioner most days to avoid drying everything out. I use Tres Semme Soothing conditioner, which doesn't cause problems.
Styling products- I don't use any styling products anymore. Hairspray especially made my scalp breakout. I have to sometimes (couple times a year) use mousse or spritz, so I haven't experimented enough to find one that doesn't cause a breakout. But if I did need to use them, I would avoid any with fruity smells and extracts to be safe.
After years of trial and error, this is what I have found to work for me. You can always take DD to a dermatologist to get help. When my Mom took me, all she was told was to wash my hair with Sebulex and they didn't stress scouring my scalp too deeply with fingernails. They also gave me a prescription for Valisone to treat the spots, but never really addressed what was cauing them in the first place. I have done better with my own experience.
Best of luck!