We had to try a number of different grains with ours, and in the end I wish we'd just skipped grains entirely and gone to, well, FOOD food. :-) For DD1, millet was the one that she did best with, while DD2 did better with oats. You may have to keep tweaking, or just remove the grains entirely, maybe try a little yogurt, which also mixes quite well with different fruits and veg, or puree soup or stew. Iron is indeed constipating, so if that's the issue you will need to cut back on iron entirely; if she's getting enough to back her up, then she's either not metabolizing it or she's getting more than she needs. At 6MO she's likely ready for a bigger variety of foods anyway, as long as she's still getting plenty of breastmilk/formula too.