adjusting multiple frame sizes in single sequence

I'm sure there's a thread somewhere on this that I can't seem to find. I'm building a montage sequence for an actor's reel taken from several source clips. The only problem I'm running into is they're all of varying frame sizes. Most are 720x480, but at least one is 624x480. When rendering the AVI sequence, ppro stretches the larger frame clips so that they appear wider than they are. I tried rendering as a .mov file, which doesn't stretch, but instead leaves the smaller clips at their original size, which puts me back at square one. I've also tried rendering the smaller clips in QT Pro so that they're 720x480, but there doesn't seem to be any change when in ppro. Is there a way to tweak either/or so that I have a single size? Thanks in advance.