The instant play option actually preserves the album order but you cannot chose more than one music track.
If you really want background noise while presenting a slideshow, you could simply start iTunes in the background, playing your playlist. Or combine all all songs from your playlist into one singe audio file, for example with GarageBand. Then use the combined soundtrack as your background music.
if you create a regular slideshow project, you can rearrange the slides by dragging them manually around, but that is a pain, if you are using one of the themes with a lot of animation. I never use them anyway. I want my audience to be impressed by my photos and not distracted by the fancy animations. So I stick with the themes Classic and Ken Burns.