Assuming you get something like this that is mechanically compatible with your guitar, or if you are skilled at this kind of construction and are able to make these mount correctly in your guitar....
The electronics part is straightforward and the piezo part is just a matter of whatever gain you need.
This might give you an idea of what you need. Build six of these (one for each pickup):
Beware of aliasing and cross-talk. It would be wise to run a digital filter, high-pass set to pass the lowest note possible on the string and a low-pass to roll off the highest (fundamental) note on the string. This will give some separation between strings up-front.
Clearly the reason for a piezo on each string is to more easily differentiate between strings, but it doesn't mean every pickup is 100% independent from the other strings. You can do some testing up front to measure how much gets picked up in the other 5 piezo elements compared to the active string, 1 at a time. This will give you some idea whether the cross-talk from other strings needs much special attention.
FFT methods as @giuliomoro pointed out will have latency than some time domain methods. I suppose you will figure that out when you get there.
Use the scope to make sure you are getting a good strong signal level into Bela. If not, then amplify the Piezo more with an op amp. Might be a good idea to follow the schematic above with a unity-gain inverting follower with the other op amp in the package. If you don't have enough signal into Bela, you can change a resistor to go from unity to as much gain as you need.