Ugh, what a hard time you're having!
I have a 19 month old who cannot tolerate dairy, soy, egg, wheat, chicken, rice (!) or banana. All of those bother him through breastmilk except banana. And boy oh boy, was it a long road to figure all of those out! ;)
First thing I'd do is eliminate dairy. Yes, do a food diary, too, but if you eliminate dairy and that's the culprit you should start to see *some* improvements in a couple of days. It'll take longer than that for it to completely clear her system and yours (took a full 4 weeks for us) but you should see some changes if it's dairy. Maybe even some big changes. Here's a great list for what to avoid: http://kidshealth.org/parent/misc/milkallergy_cutout.html And do check ALL labels... you'll find dairy in teething meds, gum, toothpaste, deli meat, sausage/hot dogs, etc. You might also pull that rice formula for a bit to see if that's the problem. It took us MONTHS to figure out that rice makes my son sick. Flu-like sick. Ick.
I would suggest that you NOT switch to soy formula yet, for a couple of reasons. 1) dairy and soy intolerance often go hand-in-hand and 2) if your daughter has multiple food intolerances it might be easier to modify your diet than to find a formula she can tolerate.
Definitely try probiotics... just make sure they're dairy free, for good measure. Try www.natren.com or www.country-life.com. They're both free of most (if not all) of the top 8 allergens. Probiotics may not help but they certainly won't hurt.
You might also talk to your doc about the Prevacid. Could be that the particular med isn't work for her. I still would go the food elimination route as well, though... I know many people who have been able to reduce or eliminate reflux symptoms for their babies through diet. (That includes my son, who used to reflux a bit when I was eating dairy, even small accidental amounts.) And don't assume the rice cereal is safe either. I've had better luck with veggies for my little guy (though I didn't really start solids in earnest 'til about 10 months 'cause of his issues). You might avoid grains entirely until she's a year or so... they're not as easy to digest as veggies & fruits.
Sorry this is so long. I've got loads of links & info... if you need more, feel free to send me a message. I'm more than happy to share the fruits of my MANY hours of research! :)
Best of luck, mama. You're doing SO right by your little girl to keep after this. Hope you can solve it soon!