Help with two input specs of a DC-DC converter
The device can have a reverse voltage applied but current must be limited to less than 0.3 amps. This is what it means. 
Adding 1 to 3.3 uF at the input will reduce input ripple current and I always do this and would probably go as high as 10 uF in some cases. Make the capacitor ceramic or film with a high self-resonant frequency. If necessary add a 10 nF for really high frequency EMI reduction.


Be aware that these are not really regulating converters - they will produce a regulated output based on the value of the input voltage but, if you change the input voltage then the output voltage also changes by about the same ratio.