If you listen to any zydeco recorded since the early 90's, you will find a bunch that used a Bb accordion. Boozoo, Beau Jocque, Keith Frank, Zydeco Force, Step Rideau, Brian Jack - the list goes on an on. A lot of these guys use other key accordions too, but Bb became more-or-less standard at that time, and continues today.
Some of the Cajun players have used it too. Steve Riley comes to mind. I have heard Reggie Matte use one on recordings too, and I know there are others. It is more rarely used on the Cajun side of things however.
Have you tried singing to a Bb? Or has the person who sings for you? I'd suggest you try it out first. I have a pretty broad range and I can't sing anything when played on a Bb. It's always too high or too low. May I suggest you consider an E? Just a thought.