[X] My Assistant

Is there a way to add more than 100 tags to the watchlist?

No, each tagset can only have up to 100 tags. You can add more tags if you create more tagsets, but at the moment you can only do so if you are a gold star member. Members above the gold star level can have up to 100 tagsets i.e. up to 10000 tags, while others have one free tagset.