Most of the problem can be solved by using the 1920X1080 frames and use the Motion effect to scale them down.
That way you can import the PAL project into a HD project later and just reset the scaling back to 100%.
But if you scale the pics in Photoshop, you will need to relink to the original material instead of just rescaling. It can be done by unlinking and relinking. Or, just move the pics to a different directory so you will be asked where they are. Then just tell the program where the full size shots are. Make sure to use the same names or relinking manually will be necessary.