Gallery shortcode, questions

I am using the <a
href=”https://wordpress.tv/2009/01/05/creating-an-image-gallery/”>WP
gallery shortcode and I have a couple of questions.

1)
I know that I can use this code [gallery id="123"] to attach as a gallery the photos in post 123. Since I am using pretty links, I don’t know what my post or page IDs are. From where do I find out my post or page Ids?

2)
In a particular page I have added a gallery of 10 photos. Then, in the same page I want to add another photo, but not inside the gallery. When I upload the new photo WP puts is immediately in the gallery. If I remove it from the gallery, WP also removes it from the media library. How do I remove it from the gallery without removing it from the media library and from the page/post? Does it mean that if I have created a gallery on a page/post every future photo I want to put in that page will be added to the gallery mandatory?

Thanks for your help!