I have encountered the a simular situation. I started out editing with DVCpro 720p 24pn footage and then some shots came in that where shot 720p 30pn (some panshots that needed to be smoother). There was also some HDV footage from a HV30.
I started up editing the 24pn footage in a 23.98 fps sequence. I then created a new DVCpro 720p 60fps sequence, copied the 24pn footage to this timeline and started editing the 30pn and HDV footage in this timeline as well. The HV30 footage was offcourse scaled down a bit, being it was 1080i and (1440×1080). I then rendered it all together in the 60fps timeline.
I am by far not a professional editor and don’t have a technical understanding of have the stuff, so this probably isn’t the best way (maybe even a terrible way) of combining the footage, but it worked ok for me. If you figure out a workflow that does the trick, I would be very much interested.