FCP7 is fine for a proxy workflow. The bigger question is whether the dpx files are the master or do you need to go back to the 35 mm film at any stage.
It is more important to speak to the post people who will be grading to make sure that your workflow is going to reconnect. Usually proxy quicktimes should have identical file names, timecode and reel number designations to the dpx files. If the workflow is xml then reel numbers might not matter but if EDL then they must. Depending on the grade tool (da Vinci, Baselight etc) there will be best workflows to make sure proxy to dpx reconnect is faultless.
There are lots of other issues like reframing, titles and VFX shots that will be complications so rather than take forum advice, get the workflow from the people who will be matching the edit to the dpx and doing the grade and finish.