Well depending on how much iron your baby has stored yes the reserves can run out at 6 mos. There are many factors including heredity and the amount you ate during pregnancy and your bodies stores. If you feel like your pedi is pushing you too hard find another. My son sees a family practice dr. I was told I was crazy but he came with high reccommendations and he is of the family is a unit type philosophy. He told me there is no reason to push solids because D showed no signs of anemia. Breastmilk does have some iron in it too. Yes the GI tract is usually immature until 6 months. In all actuality as adults we have food particles of what we eat in our stool so it doesnt surprise me that an infant does too. Usually when a baby has the front two teeth on top and bottom they will want to learn to bite off of things. Watch for things like bitting the pacifier and then yanking it out, intense stare at adults while eating, trying to eat paper, etc. If your little one shows any of these signs I would start small. It keeps my son calm just to feel like he is part of what is going on. Most importantly go with your gut. A mother's instinct is usually the best indicator for what her child is ready for.