Did you shoot 24p or 24pA, are your clips coming in as 23.98 or 29.97?
There is nothing to deinterlaced cause your footage is already progressive. If you shoot 24p and have 29.97 you need to remove the pull down.
As far as SloMo, you can try using Motion it will do a better job if you render with best (optical flow) settings on. IF that doesn’t work look at some 3rd plugins. FCP by itself doesn’t do a good job at all.