I thought I was the only one lol. I have been drinking soda to settle my stomache in the morning since I was a little kid. Last year when we were there it was not a problem to order a soda with breakfast & it did not cost extra. Although some waiters/waitresses do look a little strange when I ask for soda lol.
It won't be a problem at all. I'm never the only one filling my refillable mug with soda early in the morning either. I'm guessing there's a lot of us that gets our caffeine through soda instead of coffee.
There is absolutely NOTHING in this world better than a cold fountain Diet Coke with breakfast!!! Never, ever had a problem ordering it with any of the breakfasts we have eaten at WDW...buffet, sit-down or CS.