.mpeg .avi are containers - mpeg usually contains mpeg-1 or mpeg-2 files, . avi contains a wide variety of codecs. afaik, fcpx does not import from .avi containers, so i'd use .mpeg with a bitrate of 35.000 kbit for analogue video. if there's a prores422-option, i'd use that for editing files in fcpx.
not sure, but isn't it still possible to capture in fcpx? you can also use the .dv codec (fcpx, quicktime, imovie,...), depending on the source of your footage (e.g. if it's vhs, is should be more than enough)