Hi. We have sooo been there. My little guy spent months battling the bottle, and it wasn't uncommon for him to take 1/2 oz after not having eaten for hours, either. It is the pits...
Will they sleep feed? It can start a terrible habit, and ishould only be used if you are worried about their hydration or weight, but sometimes refluxers will do better eating while sleeping.
Our pediatric GI recommended we use 1 Tbs per ounce, but we really experimented to find the right consistency. It did take a lot of trial and error to find the right flow nipple also. Some parents use a non-cereal thickener like 'Thicken It" if the rice cereal seems to be causing problems. Not sure if your doctor has mentioned that...
How long have they been on the Prevacid?
{{{HUGS}}} Things WILL get better...