Understanding Video Formats and Attributes for use in FCP7
The codec question is the easier one: use ProRes.  It preserves quality very well and FCP likes to edit it.

As for what frame rates, image dimensions, etc. the general approach should be to go to the highest resolution of any of the originals so you aren't discarding any data.  So, upconvert to 50 fps rather than downconvert to 24 fps.  Similarly for image dimensions, conform to 1080p rather than 720p.

But I might moderate the advice above depending on what output format you are targeting.  An alternative strategy is to conform everything to a format that more closely matches the output characteristics.