I think that if you want purebred goslings, you keep your different breeds separated.
If ganders form a pair-mate bond, they will still sneak out and cheat on their spouse. Domestic geese do not pair like wild geese and you are raising the Canadas like domestic geese, so I would not expect them to form a strong pair bond, either.
If you want crossbreds, that's fine. Just don't pass your goslings off to the public as purebreds. And no, sometimes you can not sort them out just by their looks.