Hi,
Yes, you can import an iPhoto album into iMovie. You can access the photos in the album from the iMovie media browser view by clicking on the iPhoto item in the sidebar and choosing to display all albums or last 12 months of albums. From there you can import the media into your iMovie project and create your slideshow there.
Another way is to select the album's name in the iPhoto side bar and File/Export it to your desktop, and from there you can import into iMovie.
The iPhoto slideshow function allows you to set up a custom playlist with multiple songs for your slideshow. Simply click on the music icon and set up the custom playlist in the box that appears.
A simple way is to create the slideshow in iPhoto, without music, and then export it as a movie to your desktop for import into an iMovie project. You can then add the songs you want in iMovie and will have much more freedom editing them and trimming them to conform to the duration of your slideshow. You can also add titles.
Hope this helps.
-- Rich