Fermenting 5 gallons in a 14 gallon fermenter is not a problem.
Once is starts fermenting, the CO2 produced by fermentation will blanket your fermenting wort. As long as you keep the lid closed and don't muck around with the fermenting beer, you will be fine. If for some reason you were to leave the lid off for an extended time after fermentation is complete or near complete, you would probably want to add CO2 from your keg setup (if you have one) to ensure you don't oxidize the beer.
In my commercial brewing experience, we routinely put 20 bbl of wort in 40 bbl fermenters with no ill effects.
Stout Tanks and Kettles