K.A.
Have you thought about a 2nd opinion? The majority of literature today refutes putting cereal in a bottle. My son has acid reflux as well, but we never added anything to his bottle. My neice had reflux, and the dr. kept telling my sister to add cereal to the bottle until it became sludge. It still did not help her reflux - only made her overweight.
To address the amount issue, I also had supply issues with breastfeeding. However, the more you supplement with formula the less likely it is that you will keep your supply. I tried not to supplement - just let him eat whenever he wanted to from me. Eventually my supply increased to keep him happy. I'd suggest you listen to your gut -- moms know best!
Good luck!