Infant reflux rice cereal?

my doctor recommended adding 1 tbsp. of rice cereal for every 3 ounces of my 6 week olds formula due to her spitting up every time she eats...we have already tried different formulas (soy, etc.) and nothing helped..she was born with a stridor (a floppy trachea) which they seem to think may have something to do with it..but everytime i add the cereal it won't come through the hole of the bottle...i tried a stage 2 nipple and it came out but it was all coming out to fast for her to swallow and she spit it all up..is there another way i can do this to allow her to eat cereal without such a fast flow nipple? Any advice?