From what i found on internet, you can't find a perfect solution to mix different framerates (and keep the speed of your footage).
If I have some NTSC and PAL vx footage, I just throw everything in a 1920x1080 square pixels 50p timeline. It doesn't look so bad.
i filmed max's trick at 17:10, edouard's polejam at 2:35, and adrien's trick at 17:42 with an NTSC cam here :
the rest of the project is filmed with PAL cameras.
I'm not 100% sure, but i believe you should edit and export to the lowest framerate rather than the highest one