When preparing to move primary to secondary, if my volume is short of my desired 5 gallons, is it ok to add a little clean water to my secondary carboy then siphon in the primary.
Don't add water after fermentation has started, it's not a good idea. Most of the time you'll end up with less that five gal after primary due to trub loss. You can always adjust future recipe size to account for this. I usually put 5.25 gal in the fermenter if I want to finish with 5 gal.