Hi,
Weaning can seem like a daunting process with so much conflicting advice that it's not surprising that you feel confused.
Up until very recently, best practise advice was to either exclusively breast or bottle fed until 6 months of age. Now the 'experts' are debating this as new research, published this month, suggests that it may be better to introduce solids anytime between 4 and 6 months. Please don't worry that you've done your baby any harm by introducing a little baby rice at this age - you certainly will not cause 'serious intestinal damage' as misleadingly quoted above. All babies are different and have different needs and in the end it's for you to decide what suits your child.
Personally, I'd be inclined to wait until he's 20 weeks, before introducing solids, you can always give him a little extra formula. The 7oz x 5 per day is an average guide only. Also, the rice is not intended to replace his formula feed, and should be given after his bottle which at this age is the most important part of his diet. Introduce very tiny amounts in a runny consistency to start, a teaspoonful or two is enough. After the age of 20 weeks, introduce new foods, such as well mashed bananas or well cooked and pureed carrots, just a teaspoonful at a time, at one meal only and wait several days in between each new food so that if something disagrees with him you'll know what it is.
Most of all, just relax, enjoy your baby and go very slowly. Weaning can be fun and an opportunity to build the foundation for a healthy diet for life.