To create a sequence to edit multiple video formats on the same timeline (1440 x 1080, 1280 x 1080, 720 x 480), which sequence preset should I choose? 1920x1080 XDCAM EX?

The majority of video is 1280x1080.

The finished project 16:9 will be downconverted to full screen SD 4:3, so I'll be pan and scanning in a 4:3 "safe zone' in the middle of the 16:9 project.

thanks

Personally I would start with an SD timeline and use the motion effect to scale, pan and zoom everything to SD format. However, if as you said the majority of your source is 1280x1080 square pixels, that is an alternative, but just make sure you don't uprez SD to HD and then downrez it again on export. That will give lousy results. No sense in starting with XDCAM-EX settings.

i ran across this adobe video tutorial dealing with using multiple formats on the same timeline http://tv.adobe.com/#vi+f1510v1148
It says that it doesn't matter was size sequence you choose, it's only used to render the results so you can see them. the critical time to choose the size is when you export. in the mean time, all the elements are sort of "floating in the either". it isn't till you export that everything gets locked in to a particular pixel size.

I'm going to do a test to see if I choose a 1280x1080 sequence and use the MOTION settings to stretch an SD clip to fit - and then export to SD, will PPro just keep it the correct size? or will it look like it was upscaled and then downscaled, and so look worse.

Uprezzing SD material is always a bad idea. It loses so much quality, especially since the scaler in PR is notoriously bad. That is why I suggested SD sequence settings in the first place.

Their tutorial is correct when you only use ONE format in a single sequence, then paste all sequences together in a new SD sequence and scale them accordingly, but what if you have an XDCAM shot, need to insert a SD shot and later add a HDV (720) shot all in the same sequence? Then you have trouble. I haven't tried it yet, but if you try it and report back on your findings, we can all learn from your experiences.