could you introduce cereal at breakfast first to make sure there is no reaction, and then when she's good with it, then move it to dinner time? If you do this, i would get cereal in around the 5pm bf.
btw, we started solids at 5 months for exactly the same reason - ds was 'starving' and crying for food (we are formula fed). We stuck with one feed of cereal for about a month, and at six months added a second meal, and at 6 1/2 months added the third meal. It went quite smoothly, and we just added a 'snack' and pushed dinner later this past week (at 9months) because ds started with hungry nws.
good luck!
.cb.