MFCC for very short signals
Have you tried zero-padding your signals? By adding zeroes to the front and/or end of the signals you could make them longer than 300 samples without affecting the outcome of your analysis. 

Bear in mind that with very short signals your frequency-domain resolution is going to be pretty limited, and an impulsive sound such as that you describe is likely to have a very broad spectrum. The impact this has will depend on what you plan to do with your MFCCs, but essentially, there'll be fewer of them!

Using an existing package to perform the signal processing is generally a great idea, as long as you're happy to trust it! I'd certainly encourage you to make sure you understand all the processing being carried out on your data - this will affect your ability to demonstrate that your work is trustworthy.