Starting my 7 mo old son on foods and not sure how. Any advice?

He has been formula feed thus far and it is time to start him on some foods. He's hungry ALL the time and the bottles alone just aren't cuttin' it any more. I started to give him some rice cereal, first just a little in his bottles, then a little more, and now it's pretty thick and fed with a spoon. I know I can start him on baby foods (veggies first in lieu of fruits to avoid him preferring the sweetness of the fruits) and keep on the cereal twice a day for the minerals they provide. I guess what I wonder about is how much formula should he be getting a day? Should I be mixing the cereal with formula and for how long. I currently do 2 scoops of formula, 4 oz water, and enough cereal to make it thick. How long should I do this for? Also, since the formula I use (Enfamil with Lipils) has DHA and ARA in it so should I be using a cereal with DHA and ARA in it or no? I mean how much is safe? Can too much be harmful? Also, when I start him on the veggies and fruits (one new food a week in case allergies arise I know what caused it) how should I adjust the formula and cereal usages then? I just want to do it right and I'm confused. It's been 8 years since I had a baby and things are a little different now! Also, is it okay to give my baby small amounts of plain water? I'm scared to give him more than 2 ounces in a day. I just heard a story on the news of a woman using extra water in her daughter's bottles because she wanted to make it last longer due to financial issues and her baby got water intoxication and nearly died in the hospital. I'd hate for that to happen to my son because I don't know how much is okay. He just acts so thirsty while and after I give him the cereal mixture and I'd hate for him to get constipated, I just feel like the extra water might keep his insides working a little smoother. Just in case it matters, he is 7 months old, 17 pounds 8 ounces in weight, and tall for his age and he hasn't cut any teeth yet (his sister was a late tooth bloomer too. She was almost a year old when her first popped through). Currently, he takes 6 or 7 ounce bottles every 2 hours (sometimes three) with one or two feedings of the cereal/formula mixture at 5 ounces each time followed by a little water to wash it down.