You can export as 1920x1080 MPEG2 and bring this into the other project. Alternative would be 1920x1080 H.264 but I would expect the MPEG2 might give better quality as you will need to render out the section twice, it is not as highly compressed as H.264 and so may look better.
Otherwise just use one project and just apply the Theme to the photos that you want to use for the slideshow section of the project.
I like the idea of doing everything in one project, yet I’m guessing I would have an issue with doing so since the video in the timeline has been edited and the video within the available media section is the raw video (I think that is correct). However, this should work great for the stills, yet I’m not sure about the video.
My main concern is torching the non-theme video by applying a theme…
To do what you are saying; would I simply select the images (hold down CTRL and select or smart tag them?) from the media selection screen within the interface and…?
You can create an instant movie on a timeline with an existing edited movie on the timeline. Just go to Organize and select the Instant Movie icon, it will then ask you to select your assets. Just "ctrl or shift, click" to select all the photos you want included in the Instant Movie from the Organize panel and press Next... it will then walk you through the Instant Movie process. At the end the Istant Movie will be placed on the timeline after your edited movie. You can then move the Instant Movie around to where you want it.