How to choose MLCC cap values for power supply decoupling?
For filtering SMPS ringing noise, you need a snubber. There are many guides on designing such. Ridley engineering is one and it's freely available:
http://www.ridleyengineering.com/component/content/article/15-dc/127-71-designing-snubbers-for-nonisolated-converters.html?showall=1&amp;limitstart=

Here's another: 
http://cfile9.uf.tistory.com/attach/2706B1465448758E2E7E4E

If you need more thorough reading, consider buying Rudy Severn's book on it. There are various different ways of designing these, simple RC snubber is the most popular one, followed by a RCD snubber. For high power/efficiency designs you can also design a lossless snubber where the energy is stored in a capacitor and fed back into the system. 

On subject of using different size capacitors in parallel, this has a side-effect of creating an impedance spike as the inductances and capacitances of different components resonate. It may or may not matter but you're not damping that particular frequency much if at all.

edit Old Unitrode seminars have good stuff. Not to mention the way they draw basic SMPS circuits would give one our regulars a fit. http://www.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slup100