iMovie is an editing tool, not a compositing app, so no, AE is the wrong choice. If you need to do editing, get Premiere Elements/ Premiere Pro.
...and if you're actually going to be making MULTISCREEN videos, you'll obviously want all the screens to 1) play what you specify them to play and 2) all at once.
Have you thought about how you'll play them all back? Have you thought about how you'll get them to play in sync?
Seems to me you have a little more thinking to do....
After Effects a compositing and effects program. Compositing / Effects apps are used for making shots or short sequence. Premiere Pro, Vegas, Final Cut, Avid, iMovie, Movie Maker are NLE's designed to edit movies. To do what you are proposing may require both. AE could do the job but it would be horribly inefficient.
Multi Screen presentations require specialize hardware. You need to know what you are using before you complete the final project. There is no standard format for multi screen presentations. I've done about 20 of them and every single one required a completely different setup. You have to do your research on this too.