Why not just keep what cups you have? I'm not a French expert, I just tend to keep whatever came with the frame... As far as I know, other than cup threads, French and English use the same dimension spindles (at least in the Campagnolo stuff I usually work with).

Yes, there are 2-3 different diameters for cotter pins; I take one to the bike store and let them figure out the right size. The taper needs to be filed to fit correctly!

French pedal holes can easily be tapped out to British.

There are always random French and Swiss thread pieces on ebay. Be patient!

Also, unless the frame is going to be painted, there's no reason to take out the fixed cup unless you're replacing it.