Let me give you a basic frame rate tutorial.
If you shoot at 60 fps that means that you have slices one second of real time into 60 individual pieces.
The frame rate in a video file is part of the metadata and you can change that or re-interpret the footage any way you like.
The frame rate of a composition tells you how many frames per second that the project is going to be divided into. The only constant is the time. The frame rate can be anything you choose. It is a good idea to stick with standard frame rates because a lot of media players don't like it when you give them odd frame rates. When they don't like it the video does not playback in real time.
If you drop 60 fps footage in a 25 fps comp, one second of real time in the footage will take one second of real time to playback. You will not see all of the frames because they won't all fit. Because 60 will not divide evenly into 25 every few frames AE will attempt to blend together two frames to make one that will fit so there may be some quality loss.
If you have a 60 fps comp and you drop in 25 fps footage one second of real time in the footage will take 1 second of screen time to playback because time is a constant. To make this happen the 25 fps footage will repeat frames so you will see every frame twice and some of those new frames will be a blend of two or maybe even 3 other frames.
Playing back a frame twice, especially when the playback is 60 fps is not going to look bad to most folks. The old classic cartoons like Buggs Bunny were actually only 12 fps, they just photographed every animation cell twice, and they look pretty darn good.
If time is not important to you, you could always interpret the 25fps footage as 30 fps. It would playback a little faster, but you would get exactly two identical frames in your 60 fps comp. You could even set the comp to 30 fps, keep the footage at 60 to keep it playing back in real time, and set the stock footage to 30 fps. It is generally a better idea to work in even multiples of frames if you can, and there is hardly any video project that looks significantly different at 60 fps than it does at 30 or even 24 or 25.
The lower the frame rate the more careful you have to be with movement in the frame. Motion blur, which we actually see when we look at the real world, is pretty close to the motion blur you get at 24 fps with a 170º shutter (1/50 second). If we really concentrate and have good lighting we can do a little better and approximate a 1/80th or maybe even a 1/100 second shutter speed so there is really not that much advantage in high frame rate compositions when you are looking at the whole picture. The advantage for HFR video is in things like sports coverage so you can follow somebody running across the screen and have an experience more like you are at the game following the runner with your eyes. The smaller the screen the less important it is to have high frame rate video.
I hope this helps. Most of the folks that have only known worked with video do not have a really good understanding of frame rates.
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