You are fine with the choice of 24fps, it is how decks deal with the timecode for the SD proxies. In NTSC, every second lines up and the frames have a 2:3 pulldown adjustment. In PAL, the decks like to recalculate from 00:00:00:00 so 24/25 will drift 4.1% over that time.
When you bring in your 24fps EDL from FCP, duplicate the column START (cmd-D) and select TC 24. This will recreate the original 24fps. Now duplicate the TC 24 column in the same manner to the TC 25 and it will ask whether you want to copy or convert. Select convert to get the 25fps timecode as generated by the deck for a PAL down convert. Now the Avid is tracking 24, 25, and 30 so you can generate EDL’s for whatever your needs might be.
Michael
Michael Phillips