The Converters themselves are more or less bullet proof with built in protection against shorts, overload, polarity, heat and excess voltages.
But with so many of them I don't want anything else to go wrong.
Jfp.
Good. So you will have to figure out what kind of current they draw in various scenarios of abuse by an idiot with the wrong power connections. Then you will look at the data sheets and choose a part that will limit current from your power supply to a level that won't destroy the protection circuitry. If the converters are smart and draw limited current already under reversed polarity, voltages outside the spec, etc., then I'd question the value of adding a PTC.
Using one of these devices definitely requires more consideration than, say, a fast-blow fuse. Unlike a fuse, choosing a PTC is tricky--the numbers listed are really misleading. You will have to consider the whole temp range of your project, under various scenarios, and look for the trip times and trip currents. The data sheet curves are often misleading and incomplete as well, and often don't give the whole picture of performance against temperature. The trick is to keep the current low enough to not destroy the protection diodes or other circuitry. Ultimately, you can pick some parts that look close, then you will have to test them on the bench under the various failure scenarios, and actually measure the "trip time" and current. Do this at the ends of your temp ranges, and at 25 deg C. (Or, if it's informal, use a heat gun & freeze spray, or a peltier heater/cooler like Dave Jones demonstrated.
BTW, you can get pretty small SMT fuse modules somewhat smaller than an so16, where you install the socket+fuse from the tape as one piece , but then just replace the fuse if it blows.
Good luck,
Dave