What format is the footage from the 7D in? As in, what is after the . in the file name. For example, a quicktime movie can be quickly identified by its suffix, .mov.
After Effects supports a wide variety of formats, so you may not have to worry about what it likes. But you will have to export it from After Effects when you're done so you're going to want to use the format you've been using in Final Cut.
If you're done editing in FCP and you just have a single clip that you want to bring into AE, I would just place an In and an Out around the clip that you want and export it in it's original settings. So if you are working in ProRes 422 in 1280x720, make sure you export in ProRes 422 in 1280x720, or whatever the original file is.
Bring that file into AE, drag the video file into the 'Create a new Composition' button. This will make a composition in the exact same settings and length as the video clip.
Work your magic in After Effects.
When you're all done, go to Composition>Add to Render Queue (or hit Command/Control+M). In the 'Output Module' adjust the settings so that they are the same as how you exported from FCP (i.e. ProRes 422, 1280x720 or whatever it is.)
Hit that render button. When it's done, bring it into FCP, and replace the un-After Effect'ed clip.