This is complicated if you want to stay in fcp. I'm not sure how great a job premiere is gonna do (no matter what Shane says). Almost anything that automates the process is gonna limit your control and ability to get the best possible results.
I recemtly finished a feature doc with a variety of sources with different pixel dimensions and frame rates, letteboxed and 16:9 and 4:3 material. The whole thing had to be put in a 1080 24p timeline. It was then blown up to 2k and shown in a theater. I was amazed at how good it looked.
Here's some basic rules.
Compressor does a great job doing these conversions, but you're gonna need to customize your presets to get the best quality.
Prores is a great format. I recommend converting everything to prores.
If you've got material at for example, 24p (23.976 fps) and you don't mind if it's sped up a bit, conform it to 29.97 (if that's your target frame rate) using cinema tools. This just changes the speed at which the frames change, so no frames are added or subtracted. You can also do this in compressor at the same time you're converting to prores.
Make sure you turn on frame controls in the frame control panel and turn any applicable option to best. The timing options will also give you the ability to do the same thing as I described above or maintain the original duration. Again set the rate conversion to Best. If you need to convert interlaced footage to progressive, you also do it here. All these things can take a long time to do. Read up on qmaster to find out how to greatly increase your output speed in compressor. Also, you can set an in and out in the preview window to do a short test.
For scaling stuff, you use the geometry tab. This can be a little tricky to figure out. Doing short tests is the trick til you get what you want.
Post back if any of this isn't clear, or you need any further help.