You can import whole AE projects into your project, just choose them in the import dialogue like you would any other asset. When you do this the imported project will appear as a folder in your project with all of its assets inside.
What you can do is import all of the projects you've created and drag all of the compositions you want into one final composition. The advantage of this is that you can resize the comps without re-rendering the pixels.
To Illustrate: say you had an image in one of your individual comps that you'd scaled down to 50%, then you want that comp to be twice the size in the final comp. If you pre-render then you will get softness because you will be scaling up the layer and having to re-interpolate the pixels, but if you nest the composition your image will be displaying at 100% and no interpolation will be required. The downside is that it may take longer and require a lot more RAM when you render.
Don't worry if the nested comps are different sizes, you can quickly resize them to fit your output comp size by using one of three methods:
- Fit to the Comp (without preserving aspect ratio) by hitting
Ctrl+Alt+Shift+F on Windows (F for fit)
- Fit to Comp Height (preserving aspect ratio) by hitting Cmd+Opt+Shift+G or
Ctrl+Alt+Shift+G on Windows (G because it's next to F)
- Fit to Comp Width (preserving aspect ratio) by hitting Cmd+Opt+Shift+H or
Ctrl+Alt+Shift+H on Windows (this should be the shortcut for fit to Height, but it isn't)
You can select all of your layers and do this in one hit. This is great when you have a bunch of layers with different sizes.
Once you've done that you'll want them all to be one after the other. Select them in order and from the menu menu choose Animation > Keyframe Assistant > Sequence Layers (or if you right-click on the layer choose Keyframe Assistant > Sequence Layers).
You could also investigate turning on Collapse Transformations/Continuously Rasterize for the nested compositions. It's the icon on the layer switches that looks like a sun. What that does is to do all the transforming - rotation, scaling etc before rasterizing the layer, meaning you won't get pixelation on any shapes, vectors or text etc in those layers.