Once scaled to the desired size, using the SCALE or MOTION, save those settings as an effect template. Give it a name. This becomes just another choice, at the bottom of the effects configuration window, that can be applied to any clip
One approach to lining up the scaled videos is to use the base X and Y sliders in the MOTION effects dialog. Jot down the vertical (Y) number, for instance, and use it for every video insert.
Place each on its own track, keeping in mind that higher tracks will be the picture-within-the-picture. Track rearrangement should be minimal if needed at all; most pics will occupy a specific area of the screen. But, If an overlap is desired, just remember that higher tracks take precedence over lower tracks.
If you make a mistake, press <ctrl-Z> to step back, edit-by-edit. <ctrl-Y> steps forward. This is the same as EDIT | UNDO and EDIT | REDO.
Back up the project often, using SAVE PROJECT AS. Give it a unique # or name, so as to not overwrite earlier saves. SAVE PROJECT (no "as") will overwrite older projects.