Hi Larry,
Since you're mixing 3 different sizes of video, the desired output DOES play an important role from the start and will help determine what Sequence Setting you want to start with.
You have 720p HD videos, SD videos, and "less than SD" video. I would recommend against exporting as 1080 HD unless absolutely necessary for some reason, since you would be "blowing up" ALL of the formats, and that loses quality. The 720p would look ok, but not the other ones!
So at most, I would edit in a 720p30 sequence for 720p delivery. In the Premiere Preferences, there is a checkbox for "Scale to Frame Size" - check that and then any clips that you import will automatically fill the frame you're working in. For clips imported prior to that change, right-click the clip in the sequence and check "Scale to Frame" manually per clip.
Another example, if you wanted to export to DVD which is 720x480, then you would edit as 720x480 so that the SD clip would be edited natively and the smaller clip would upsized a minimum amount. The HD clips will downscale fine. That would yield best results for SD delivery, rather than taking all clips UP to 720p or 1080p, then back down to SD again. Following the workflow?
Thank you
Jeff Pulera
Safe Harbor Computers