Are you using iPhoto '11? I tried in iPhoto 9.4.3 and there you can set individual timing for more than one slide at once. Select the slides in the timeline and open the "Slideshow Setting > This Slide" tab.
Set the timing you want to use. It will only change the timing for the selected slides.
2. When I play back my slide show it hangs up after about 45 photos. I have a MacBook Pro running OS 10.8.4 with a 2.2 GHz and i7 processor and 8 GB of ram. Should it being doing this? Is there a better way to preview my slideshow before I export it?
It should not hang.
Is it always the same photo or movie that makes it hang? If yes, what happens, if you remove the photo from the slideshow?