I don’t know about photoshop elements.
also, i don’t think fcp cares whether your photoshop file was square or rectangular pixels, i don’t believe it can read that information.
bottom line is, if you import a graphic file with native codec dimensions, such as 720×576, it will match the video size and won’t have to rescale it. if you import a 4×3 image, fcp will have to rescale it. it is generally recommended to pre-scale the graphic files before import (someone please correct me if i’m wrong). whether you work with non-square or square pixels in photoshop seems to be a moot point. the question is, which resizes better, photoshop or fcp? frankly, i’ve never quite noticed, or cared to notice the difference.