What is your OCS date?
No matter what, you will get really accustomed to eating rice and potatoes. Every flippin' day.
As far as beverages go, you're pretty limited to water, gatorade/powerade, and milk for the first couple months. Not sure which month you're not allowed dairy . . . I've never heard that. In fact, the OC regulations state that milk will be available at every meal (not necessarily the case if they run out).
Everyone is encouraged to purchase daily multivitamins, but beyond that I suppose it would depend on your class team.
You can write special request chits for just about anything- especially if you have a legitimate medical issue and/or prescription.
Hope that answers your questions. PM if you have more.
Funny story about the chow hall powerade. So when I was a candio, the class team required that every IC get one glass of powerade and one glass of water at every meal. Also, it was part of eating by the numbers to completely finish all of their fluids. One meal several IC's get up with the usual "Sir, this IC requests permission to speak to a candidate officer! This IC cannot drink this powerade!" So I tell the first couple basically to suck it up and finish their fluids. After the 3rd or 4th denial I saw the rest of them getting smart and pouring their powerades into their fruit/salad bowls. I also noticed the "powerade" bowls were filled with a deep, dark blue/green fluid. The light blue powerade mixture apparently was very heavy on the syrup that day.