But please consider quality,
FCP is not a format conversion tool, so decide what your final output format will be and convert the other media to this before editing.
Consider the quality of the conversion also, some of the s’ware converters look ok but blow them up to delivery size/codec and then judge. IMHO the best one is in Compressor2’s optical flow technologiy, VERY slow, really only useable in an Intel Mac.
Same with 16:9 and 4:3, it’s a bit like circles and squares, you can do it but can your client accept the result? So which rato do you want/need?
Depending on your delivery ( test for quality loss), to avoid the black bars, you can scale and cropp the 4:3 to fit into a 16:9 sequnce, but you already knew that. Not bad for web or DVD. but ? for other formats.
Cheers
Alexander