Thanks for the query.
Its normal for the child to pass stool immediately after a heavy meal and sometimes after every heavy meal. It is called gastro colic
reflex and there's nothing to worry about.
There is no need to change the cereal. As long as your child is gaining weight normally and passing normal stool and urine, you are on the right track.
Now that your baby is 6 months old, you have to emphasize on and give different cereals, fruits and vegetables gradually and slowly introduce different tastes, so that by 1 yr she has milk only 3 times a day and is able to eat regular food. So,for your second question, ensure, she has a semisolid cereal meal rather than a bottle as she needs it for her rapid growth.
Hope you are reassured.