On the advice of our pediatrician, we started giving our kids solid foods after 6 months. When she told us how to do this, she indicated that the quantity of solid foods wasn't particularly important, because the baby was still getting its primary nutritional needs met by the breast milk (in your case, formula). Our kids weaned on their own anywhere between 9 months and more than a year and a half, depending on their preferences.
First, I recommend asking your pediatrician your questions, as sh/e will be able to provide you advice based on your child's medical history and needs. It sounds like your child's needs are different than the average child that is the same age, so your child's doctor is the best place to get definitive advice.
When you decide to start on solid foods, our doctor recommended that we start with single-grain cereal to start with. We started with rice cereal. We then added baby food (either commercial, or mashed up on our own) one flavor at a time. This is especially important if you are concerned your baby may be affected by allergies. If you see an allergic reaction, you want to know what caused it. That means you should only introduce one new food every couple of days.
Finally, my last bit of advice is that if your child rejects a particular type of food, don't consider it a lost cause. You may need to introduce a food to your child 10 or more times before they decide they like it. Persistence is a virtue.