Footage with different aspect ratio
One way to deal with different aspect ratios is to take footage that is not as wide as your output format and duplicate it in another layer. Increase the scale on this other layer and blur the image, then put the original over it. This produces a nice coherent effect where the area outside the original footage is filled with colors that are from the original footage, but are not distracting.

If you're working in FCPX, there's a built-in filter called "Photo Recall" that does this sort of effect. It shrinks the original image, but you can have it fill the frame instead. Other apps probably have a similar filter or make it fair easy to reproduce.