I'm making an animation within a 30fps composition, but it uses various pieces of footage and animation I have filmed/created over the years.
Most of that is also 30fps so that's not a problem to work with in the composition but I also have a couple of GIFs which are 15 or 10 fps that I would like to use in the animation. I've always been led to believe that it is not good to combine footage of different frame rates, however the GIFs (which I have looping in the composition) look fine to me so far and after I render the final video with the 30fps setting. What are the problems that may arise for me doing it this way? What are the alternative options that will allow me to use these GIFs in the composition?
[Keith Kelly]"I've always been led to believe that it is not good to combine footage of different frame rates"
Various NLEs like FCP over the years have had less-than-stellar support for mixed frame rates.
After Effects doesn't care what frame rate your media is.
The only thing to note is that where there is a frame rate mismatch between a source and its containing comp, it must be resolved -- either by duplicating/skipping frames as necessary, or by synthesizing new frames.
If you're happy with the look of your animated GIFs, don't worry about it.