As I explained above, the image ID number is gotten from the image insertion code or by looking in the Media Library. You won't find it in the photo's caption. Refer to the post I gave above. http://en.forums.wordpress.com/topic/new-gallery-can-you-put-in-single-photo-without-it-going-into-gallery?replies=11#post-1031891
I have a feeling you may have copied my above gallery shortcode and pasted it in your Post Editor's Text tab, along with the shortcode that was already there. That's why you ended up with two galleries. Don't copy/paste. You have to modify the existing shortcode to exclude the single image.
If you want all your Galleries to appear as mosaics, it may just be easier to set it as the default display. Here's what the Gallery support doc says
To switch to tiled layouts from the default, head over to Settings → Media in your blog’s dashboard, scroll down to “Image Gallery Carousel,” and select the box next to “Display all your gallery pictures in a cool mosaic.” Note that choosing this option makes this layout the default for all your blog’s galleries, including those you’ve previously inserted.
You'll still have to use excludes if you want a separate image to appear in your post that contains a Gallery, but following the instructions I gave you above, you should be able to find it.