I was trying to create a new sequence that does not use the specs of standard DV or HDV and tried to configure it to 640x480 QT using Sorenson Video 3 Compressor - thats the material we want to import. However I did not get an option to choose Sorenson 3 for that sequence.
When exporting, Sorenson 3 is on the list of Codecs to use, so it is on the Computer.
Sorenson 3 is a delivery format, not an editing format. My suggestion would be to import into AE and then save out uncompressed files to work with in PP. This will give you the best quality and least hassle.
Don't worry about it. Your choice for the Preview file format and codec only affects the rendered preview files - it doesn't affect the edit or the export unless you select the option to use the preview files in the Export settings dialog.
If you do choose to use the Preview files, it's better if they are as close to uncompressed as possible. MPEG/I-frame Only is a good choice. Are you on a Mac? Is QT/None or QT/Animation or QT/Photo-JPEG available as a Preview file format/codec?