I gave my son a ton of formula and his tummy was always fine, we got lucky there. Of course he had formula from birth as the nurses gave it to him while he was briefly in the NICU. I guess they weren't too concerned about getting breastfeeding going. And he weaned off breastmilk to just formula at seven months. My daughter never had formula though, and I breastfed her for two years! She is a much more adventurous eater now than my son, I always wonder if the no-formula made a difference (some experts have said that breastfed babies are exposed to a greater variety of flavors so that might show in their palates later).
Wow- two years, that is awesome! Most of my kids have gone to formula or breast milk that I had stored up around 7-8 months. My little one now is almost 9 months and still nursing- I feel so accomplished for making it that far, lol!