Fizzy drinks is another one of those issues that people are divided about...the theory behind not drinking fizzy, is that it can cause your pouch to stretch if you are gulping down large volumes of carbonated fluids.
Fluid is fluid though, so I can't see how fizzy is more likely to force food through the stome than normal fluid, although in principal, drinking any fluid for around 30 minutes before after eating can indeed flush food through the pouch more quickly.
I personally know someone who drinks a lot of de-caf coke...but has lost about 14 stone to date & is still losing.
I'm not advocating it one way or the other, but I think it's another one of those personal calls. I don't think an odd small glass of something fizzy, left to go flat for a while will hurt you. Why not partially open a bottle of lemonade/soda water & let it go completely flat, so you have something to hand if you fancy a little tipple?