Hi Jenn,
There are a couple of ways you could do a single random image. 1) Use the PhotoSmash Widget...just set it to a Random gallery and show only 1 image. 2) You could use the shortcode...this off the top of my head, so let me know if it doesn't work: [photosmash gallery_type='random' images=1 layout=my_layout]
What you'd want to do is create a Custom Layout (have to have Advanced settings turned on). If you set the Caption Settings to link it to the related post, it will link to the Post where its parent gallery is. This is all very doable...let me know if you have questions.
For your second question, you can't have an image in multiple physical galleries, but you can have them show up in Tag galleries as well as Contributor galleries, in addition to their natural gallery. So, I'd recommend tagging the ones you want with a tag and then running a tagged gallery where you want to show the public images. You can set the tags in Photo Manager. A shortcode like: [photosmash gallery_type='tags' tags='my tag' layout='my_layout']
You don't have to use custom layouts, but you can if you need something special.
Hope that helps.
BB