Array wiring

Specs for the panels: 255 Watts

Max Power Voltage (Vmpp): 30.5 Volts
Max Power Current (Impp): 8.35 Amps
Open Circuit Voltage (Voc): 37.7 Volts
Short Circuit Current (Isc): 8.74 Amps

I usually recommend higher voltage to reduce voltage drop. The MPPT charge controller will drop the voltage to match the battery bank and increase the current, so the volts and amps on the output will be the same regardless of how you wire it. The advantage is you can use smaller wire with the higher voltage.

If you are doing 3 nominal 20V panels in series, you will have 20V x 3 = 60V nominal in / 24V nominal out = 2.5 ration of voltage dropping and current increasing.  So if you have 4 parallel strings with 8.74Isc x 4 strings x 2.5 ratio = 87A out. NEC says you have to add another 25% when selecting a charge controller, so you would need a charge controller that can handle 87.4A x 1.25 = 109A. I don't know of any that can handle that, so you will need to break it into 2 circuits, with 2 strings of 3 going into each charge controller.
8.74Isc x 2 strings x 2.5 ratio=43.7A x 1.25 NEC requirement = 54.6A minimum charge controllers, so probably two 60A charge controllers needed.

Note that if you were doing a 48V battery bank, you could use just 1 charge controller. 60V in / 48V out = 1.25 ratio.  8.74Isc x 4 strings x 1.25 ratio x 1.25 NEC = 54.6A charge controller for the whole system.

The FlexMax 60 can handle 6 of the Canadian Solars, the FlexMax 80 can handle 8 of them. The 6 you could wire in 2 parallel strings of 3 in series, or 3 parallel strings of 2 in series, either way, you are right, with 1/0 cable, voltage drop shouldn't be a problem. If you switch to the FM80, the 8 can be 4 parallel strings of 2 in series.