Hi,
Interesting to see articles you refer to. That's a big topic, worthy of study itself. Not sure if will help, it's about LC filters at input to SMPS, since 'An SMPS presents a negative impedance to its source supply.'
Some to look at if you want:
Power Tips #3 Damping the Input Filter (parts 1 of 2 and 2 of 2)
snva801 Analysis and Design of Input Filters flr DC-DC Circuits
SLUP340 SMPS Compensation Made Easy (p.31 - p.32, Input Filter Stability)
There are lots of online tutorials and application notes about SMPS input and output filters that can help.
If you just want a capacitor at the input, look at RC time constants, permissible inrush current or di/dt, (linear) soft-start circuits... You need the capacitor to hold up the input voltage for sudden step loads, same as output capacitance, I can't remember offhand document names but 'calculate bulk (input) capacitance' should bring up some useful results and I'd expect a good few online calculators.