I've also read sources encouraging them from 6 months as meat is a very good and very absorbable source of iron.
If you're going to go down the traditional weaning route, I would cook the chicken with the vegetables in some water, it's then softer and will puree more easily. I think I used to do something along the lines of Annabel Karmel's one pot chicken, basically onion softened in unsalted butter then add veg, e.g. sweet potato, butternut squash, carrots and some thyme / parsley or both and chunks of chopped up chicken. Add water or very low salt stock and simmer until everything is cooked. Puree.