Both the PDB breakers and the main breaker are thermal, meaning that they heat up as you put more current through them, and when the reach a certain temperature they trip. The main breaker is rated for 120 amps (not 240), but it will not trip immediately if you exceed that value. For example, at 240 amps, it will take anywhere from 10 to 40 seconds for it to trip.
See the datasheet for more information. The PDB board breakers are the same way, but the trip significantly faster.
Here is their datasheet.
As for having eight drive motors, you could do that but they could not be all CIMs because the rules limit us to using only six (this year). But you could use a CIM/minCIM pair (we are planning on doing this in the offseason).