Any smaller sail can work on a bigger board, it just will be outside its comfort zone (the opposite doesn't hold true). 5.0 sounds just a little too small for a 114 and smaller yet for a 122, but might be just the ticket for very flat water or very gusty conditions where the holes were big. I'd put a smaller/more wavier fin on though. Generally, though, by the time you have enough wind to power a 5.0 at most venues the water will be choppy enough that the big board will have a harsh ride, and may be difficult to control from the air getting underneath (not like a really big board though).
I've only sailed smaller combats at demos, don't own one, so hopefully you'll get better than my general advice.