This has nothing to do with being London or Paris. I'm not that shallow. You did, however, address my point in your post: if TC vehicles are too large to corner a legacy streetcar curve, they won't be able to travel on the old streetcar network to begin with. Therefore, why bother sharing the same, obscure track gauge?
Actually, it hasn't come up in discussion yet, but just as importantly, what voltage do we want TC to use? The 600 V DC that is used by the TTC, and only the TTC, or the 750V DC that practically every other light rail operator in North America currently uses?
Although the legacy network uses a specific track gauge and voltage, it would be much more flexible if Transit City used a standard gauge and voltage. For one, it would allow surplus vehicles to be acquired from another city with much more ease.