I agree with everything that Nick said.
However, if you want to do the random thing, just create the slide show in iPhoto and then export a quicktime movie of the appropriate size. I don't spend much time in iPhoto – it may be under share instead of export – however I have tried this in a pinch. It was very fast, but less than satisfying. I ended up doing each move and transition manually.
I prefer to build video slide shows in After Effects or Motion. FCP key frames are not as good as AE keyframes, and behaviors in Motion make this kind of thing pretty fast if you don't need precision.
I just re-read your post and realized you are asking about transitions only, not motion. Not sure if iPhoto will give you random transitions or not.
Best of luck.
Tom
Message was edited by: Tom Meegan