I agree, ProRes is always great on a Mac but do be aware that as ProRes is a codec is does slightly slow down playback as After Effects has to decode every frame. If you use an uncompressed image sequence like TIF, After Effects will playback faster, if you are stuck for drive space then PNG is also an option.
Regarding the frame rate change, if you wait until export to make the change After Effects will effectively remove every 5th frame making playback of moving shots (particularly noticeable with camera moves) a little jumpy.
It’s always better to handle frame rate conversion in the Comp. If you are just using animated elements then changing the Comp frame rate is quick and simple. If however you are using footage that needs to be converted there are several approaches to that depending on if you have synced sound or not in the footage. IF no synced sound then in the Project Panel, select the footage, right mouse button on it and select Interpret Footage, from here change to the desired frame frame and then make sure the Comp is set to the same frame rate.
Hope that also helps.
John Williams
Soho Editors