Generally start with one say, 0.1uF capacitance, and if it doesn't work, try changing the value lower or higher (depending on the frequency you need decoupling at) - once that is ideal, the best thing to do is add more of the exact same value capacitor in parallel.  this increases the capacitance but also DECREASES the paraistic inductance, so it gets BETTER and has more decoupling capacitance.

note : People used to add say, 0.1uF, 10nF, 100pF caps in parallel because in theory this would give a wide range of decoupling

the problem is it produces anti-resonance where the coupling gets worse.  it's a very, very tricky thing to try and fine tune, and should be avoided in 95% of cases.

my suggestion - put pads for lots of 0603 0.1uF capacitances, but only populate the reference PCB amount.  if you need more or have to tweak, the pads are right there for it.  It's standard practice to have pads for parts you don't actually populate going into production.

I think you could easily adjust the core voltage with some sort of programmable resistor on the buck reg. Not sure if such a thing is readily available but it should be. You could probably use a few FETs shorting out a binary series of resistor values to adjust the voltage divider.

eDiT: Oh geez, here you go...

http://ww1.microchip.com/downloads/en/DeviceDoc/22107a.pdf
(Now that I think about it you could probably use an analog output from the PIC as control voltage on the regulator but that would take some digging into to figure out)