When you scale using the grab handles, holding down the shift key keeps the scaling proportional.
I looked at your sample.
If I could make a suggestion, those stills are hard to look at because they are all muddy. When you pick the frame to still a guy on, freeze/export that still into photoshop. Duplicate it into layers, enhance the contrast. Cut the guy out of the top layer, add a gaussian blur to the under-layer.export the psd file or a tiff version with alpha preserved, back into FCP preserving all the layers.
Now, if you make it B&W or keep it color or tint it, you have faked the tight depth-of-field “look” of a still photo shot, which was what I believe you were going for there…
Putting a subtle zoom-in and/or pan move on the top layer cut-out of the guy, when it’s back in FCP, adds some extra “zazz”. Make just the background layer B&W, and the guy in color… You could also strip out the top layer and lay it over white or black or a gradient or completely different backdrop for the fake still composite. None of this is really that much work to do.
Looks like a fun project to work on, best of luck.