I would think using an 18v that was discharged to the point that it wouldn't run the gun would run a 14.4 just fine, but I'd be skeptical of damaging the pack by running it down that far. For sure I wouldn't put a freshly charged 18v in the 14.4v gun. Might work for a while, but when the magic smoke comes forth, then you'll know for sure. I've never ran a 14.4 or an 18, do they have a variable speed trigger like the Dewalts? If so, you might be able to get away with it.
:my2c