I had a very similar problem. My son Noah has severe Reflux/GERD and had swallowing studies done when he was only 6 pounds. They put him on a couple meds (one of which he is still on) and also had us thicken his formula (when I was breast feeding I thickened it with a gel I got in the NICU that was safe to thicken breast milk with). We started with rice cereal and ran into the same problem. Our neonatologist and pediatrician both suggested switching to oatmeal or barley cereal as they have more fiber. It really helped. Sometimes we would also add a little prune juice to his bottle as well. He's almost 20 months now and still has it, but I am hopeful he will get better without requiring surgery.
Good luck and hope she continues not vomiting! :)
Mom to Noah 2/17/03 (born at 26 weeks, severe PE)