MC,
Kevin told you step-by-step how to do this. It’s really simple. No zoom or pan on a shot, 72×480 at 72 dpi. If you want to zoom or pan, 2x or 3x as big, 1440 wide or 2160 wide.
Your original scans can be much bigger, but you optimize them in Photoshop to the above sizes. That’s it.
Second question, just do everything in video at 72 dpi. Period.
If you want really smooth moves with no keyframes, I highly recommend PanZoomPro at lyric.com