Doug,
I had the NG Tube as well, and when I had it, it was "one size fits all". I was made to understand that they have a pediatric sized NG tube these days. Not sure if that is right or not. I didn't think they would fit the adult size into my nostril because it's very tiny, and I also have a deviated septum secondary to breaking my nose while playing softball. But they got it down pretty quickly. But it's something you should ask in case you need the dreaded NG tube again, which unfortunately down the road you might, and if it's an emergency down the road due to blockage or whatever you should know what other options are available, sooner rather than later.
Regarding the gas issue, I like the suggestions of @Jaydog and @Kmiller posted above. It's probably premature for you to be taking anything like Pepto Bismol, which I would recommend further out from takedown as it crushes these kinds of issues but also has a constipating effect. I think having a lot of gas after takedown is somewhat normal. In terms of what you can take orally or medicinally, you should talk to your treating surgeon.