I would go for the mediation first, just to be on the safe side. Point out that there are other by-laws that would allow them to remove the dog if it turned out to be noisy. The EC can’t assume there will be a problem because not all breeds are the same and you’ve tried to find one that won’t be noisy. This is a clear case of the EC not reading their own by-laws and then not liking what they see when they do. But you can argue that you bought your apartment on the basis that the building allowed pets so they are the ones in breach, not you.