It's so all over the place, isn't it? We just have to use our medical team's advice and our best judgment, but it's tough when people get different instructions.
Our surgeon didn't give any advice, it all came from the nutritionist - she told our group it wasn't because of stomach stretching (which honestly doesn't make sense to me - the bubbles go in, you belch, they're out) but because of discomfort. She said that some patients find it more comfortable to use a SodaStream with a lower amount of carbonation, or to let it sit for a while before trying it, but either way to leave it alone for at least the first few months. I learn slowly, so I've tried it already and it wasn't terribly comfortable unless I let it sit in my mouth for a few seconds for the fizz to go away (which, it turns out, was really all I wanted anyway - bubbles on my tongue - and now that I've had it I'm over it and don't want anymore).