It's important to remember that flash files are not video files. They are built for the internet and run using vectors, not pixels.
That said, I had no problem dragging them into FCP 5.0.4 and opening them. (I did have the original .swf files.) They turned out to be half the size of the screen, though -- which was fine for my project.
FCP will not play Flash Audio. You have to capture that independently -- try Audio Hijack -- while playing the original flash file in flash.