Here's a user tip I wrote about mixing frame rates in fcp
Unfortunately Apple saw fit to rearrange the forums and make the user tips difficult to find.
If you need any further help, post back.
very good article by michael.
some words i would like to add: your master determines your framerate. if you need PAL, it is 25 fps. if you need NTSC, it is 30 fps. if you work for a computer environment or streaming, it is 30 fps (even in PAL countries).
if you need to convert PAL to NTSC or vice versa, FCP can do it by blending frames. but it is not high quality. a professional standard conversion is way better (smoother motion, less artifacts). but it is not cheap. find a video studio in your area.
best quality you can achieve on your mac is reconforming the material in compressor, ie telling the files to play in the desired framerate (so there will be no frame blending at all, ie no artifacts). a PAL file then plays with 30 fps, so it is 16% faster. this may be acceptable for some footage, for some it is not. you may want to repitch audio (which will play in a higher pitch, too). NTSC at 25 fps will play 16% slower, which gives a nice laidback effect. audio pitch is lower. check whether this works for your footage.