I would go with which ever codec has the largest amount of footage. Say 75% is 29.97, 10% 25fps, and 15% 23.98, I would make the project 29.97.
As for the conversion, I often use JES Deinterlacer for this kind of thing. While not perfect it does a very good job at conforming frame rates. Its not as easy to slow the footage down because you will get frame stuttering and it wont look that good.