No reason to shoot 60i. 60i should really have never been invented (only reason it was is because I believe the only thing tapes know is 60i, which is a different situation). Might as well shoot 30p. You can de interlace 60i, but by doing so (assuming your saying 1080i60), you down res your shot. You actually create what would then be like 1280x400 (varies depending on camera and codec involved) thats then converted to a psuedo 1080p60. Pretty useless since at that point you just went through steps that took time and created something slightly lower quality then 720p60.
Of course if hes working with a cam that only does 1080i60 or 1080p30, then I would say shoot the 60, but only if you know your going to slow it down (you can just convert it to 30p with simple pull down, which is what the camera does anyways to achieve 30p if you dont want to slow it down in post, but thats time consuming). 30p is going to look a hell of a lot better and crisper then 60i ever will for a standard shot.
Now onto your next problem. I dont know what your cameras whole pull down deal is (especially since I dont think you've mentioned what camera it is), but you shouldnt be shooting in "24p" unless the camera extracts the un-used frames by 2:3 3:2 pull down for you. Well 24p is awesome for crisp shots, terrible for skiing, and if you dont know what your doing the hole pull down process might be a pain (i really dont know what the pull down technology is these days).
Now that you have that 24p footage, you cant manipulate it very much. Twixtor is a great plug in if slow mo is a must, but twixtor is a bitch to use since it doesnt change your clip lengths to adjust.
Someone above me said to use optical flow in motion. Yeah that works, but the optical flow doesnt do a very good job in my experience. It will make the shots slowed down and smooth but it blurs a lot of frames from my experience. AEs optical flow has tended to work better for me. I still dont think either are as good as twixtor, but at least the optical flows change the clip lengths.
Buy Collab now and get Overlooked Free!