16:9 and 4:3

I recorded video on two different cameras. I suppose one is 16:9 and one is 4:3 because now that my footage is in a quicktime movie, half of my video looks fine and the other half looks stretched. What should I do? Is it something about making it anamorphic? And can I do this once the footage is already in the timeline? or will I have to adjust it in some way and drop it into the timeline again? I'm using Final Cut Pro 7.