FCP EDLs are ok for simple cuts and disolves.
they dont handle speed changes very well at all.
we were using EDLs recently at the film school im working at.
speed changes came across as a frame rate, the machine wanted a percentage.
so we opened the speed mod clips' item properties, and included the FCP % in Master Comment 4, which is the defult EDL note
might have been the format of the EDL, (Sony9100)
maybe the others use %, i dont know.
as for best format, talk to the online house.
CMX3600 is the most widely accepted, IMO
include lots of notes in the EDL describing the Speed changes, if you have them.
of course you have to colapse your FCP sequnce to as few tracks as possible.
make a separate EDL for each Vid track.
or..
talk to the on-line house.
maybe they have automatic duck, and can make an Avid version of your FCP timeline?
maybe they have FCP?
if so, i;d sent the FCP project file over, as well.
cheers,
nick