Confused on baby food amounts

Hello..

I have a 6 month old son who was 6 weeks premature so he is 4 1/2 months gestation. He has just started eating pureed foods a couple weeks ago, and I'm very confused on how to convert to the purees from rice cereal.

Do I keep the same amout of formula and substitute baby food in place of the rice cereal gradually, or do I just give a few tablespoons of puree and give mostly formula and rice cereal, or do I cut back on his formula and substitute it (gradually) for puree/rice cereal??? Simplier put, Im very confused on which should be his main source of nutrition right now, and which I should be weaning him off of.

Also is it possible to upset his stomach by giving him new foods too fast? I've given him 3 different veggies in the last week and so far so good, but can never be too careful.

P.S. He has had a sensitive tummy in the past to regular formula but nothing other than that.