they are 5 US gallons, if you don't vent the CO2, at best the yeast will die from too much pressure and at worst the keg will go kaboom.
You can very easily work with a standard airlock, although, I'd use a blow-off hose during active fermentation.
Basicallly remove the gas-in post, take the poppet out, replace the gap with one dip-tube-sized o-ring, and put the post back on, then connect to blow-off hose. If you prefer an airlock, take post off, screw a tight-fitting hose that is about 1.25 inches long over the MFL threads, put air-lock in top of hose, and tighten in place with a hose clamp. Worked very well for me.