I understand that when overcranking (slo mo) the footage is reduced to 720p. If you shoot the rest in 1080p, is the overcranked footage expanded to fit the 1080p footage?

No. Digital cameras with solid state storage can store files with different properties. The files are stored as 1080p and 720p.

When you transfer the files from the camera to your computer, they retain the properties they were shot at (e.g. frame rate, resolution ...). Of course, when you edit your clips, you can stretch and shrink them however you want.

If you're asking what happens when you play your video from your camera on a high definition component video TV screen, I don't know exactly what would happen but I would guess one of two things would happen:
1) The camera scales the video as part of the conversion to component
2) The component video signal will switch to 720p and your TV will detect that and change to it's 720p mode

If it's a standard def TV, all the video will be down-res'd to composite and it will all be full screen.

NOTE: Overcranked footage does not have a sound track.

Normalizing video clips in different standards/formats/specifications is typically done during editing. You select some standard for your production (1080p as in your example), then whatever you bring into the timeline will at some point be up- or down-converted into the production standard.

You didn't mention what video editing tools you are using, but some require conversion (trans-coding) before you drop the clip into the timeline, while others are more tolerant of non-standard video. But ultimately, every clip must end up in the native format of the production by the time of the final output rendering.

Some editing tools do this (~covertly) for you in the background. But you pay the price of having some automatically-selected method being used on each clip which may or may not be optimal for that content. Consumer-oriented tools tend to do things like this automatically, while more professional applications require manual attention to non-standard clips (but while offering better control of the conversion).

At the playback end, indeed, you will note that HDTV screens have various native resolutions. I have some that are true 1080p, but others that are 1366 x 768 or even 800 x 400 (small screens, etc). The displays themselves (or sometimes the disk players) convert the video on the fly. They take the incoming resolution and interpolate it up or down (as necessary) to ultimately fit the native resolution of the screen. True aficionados of video insist on using at least 1920x1080 screens to avoid the artifacts of interpolation when viewing video of the highest resolution.

Using Premiere 5.0. Just wondering if you drop a 720p footage onto a 1080p timeline, will the 720p footage not fit the frame? If not there is a "fill/shrink to frame feature in PP. Is the resolution noticeably reduced?

There is a way to find out: try it ;-)

720p is 2/3 of 1080p so the footage lacks resolution.
Resizing it, is not the problem (some NLE's do this automaticly, some don't); the challenge is whether people will notice the resolution/ImageQuality difference.

With Red Giant's InstandHD you can 'uprez' (blow up) your 720p footage to 1080p.
I find that this way it looks sharper and more natural than when you use normal scaling in your NLE.
But this is something your should try and test to see whether it looks good enough for you.

Ahh. Thanks for the tip. Just started doing HD, so I wanted to find out these things rather than muddle through it.