This is odd behavior, so something else is likely wrong.
When you Import the video-sized PSD's and they do not fit properly, I'd first look to my Project Preset, to make sure that I had used the correct dimensions and PAR, when I did my .PSD's. That could be an issue and a simple check of the Project Preset will tell you.
As for the AI files, PrPro will Import them BUT, it will Rasterize them, and PS does a much better job at this. For all AI's, I do the Rasterization in PS and Save_As PSD's for Import into PrPro. This FAQ in the PrPro-Wiki addresses some of that.
Now, if your PSD's DO match your Project Preset, please give us the full details of both your Project Preset, any settings, like Scale to Frame, and the full details of your .PSD's.
I have exactly the same problem and I'm convinced its a problem with Snow Leopard (SL).
Prior to upgrading to Snow Leopard, I made an exact backup of my Mac HD. I created a project on SL using the latest CS4 on a secondary HD. When I imported an AI file I got the same problems you experienced including multiple crashes. I was able to save the project with the AI file.
I then booted to may backup drive running the previous version of OSX and the same project opened and worked as expected, no problem at all. Since I am on a deadline, I have not booted back to SL yet.
I hope Adobe is monitoring this and will come up with a fix SOON.
If you have an old bootable backup, you can probably get your project running.
My system is a MacPro 8-core 3.2Ghz with multiple drives, 16GB ram and Nvidia 8800GT.
Welcome to the forum.
There have been many folk complaining that Adobe programs (not just PrPro) will not run, or will not run well, on Snow Leopard.
Apple is supposedly working on an update to fix much of this. Being a PC-guy, I have no idea of whether this is true, but given the number of Adobe programs on the Mac, it would be a big issue for me.
Good luck, and let's hope there's a Snow Leopard II in the pipeline soon.