thank you very much for your reply.
FTM I am only able to use 2+2 motors by a Pololu motor board plus 4 NXT touch plus standard i2c.
The Pololu HAT is using pins 5,6,12,13,22,23,24,25.
For rotary encoders I have configured pins 17,27,19,26.
dPins für touch are 18,16,20,21 (configuration optionally altered randomly)
SPI is disabled so far, nonetheless Pololu uses SPI SS 25 for pwm.
So I certainly can drop the Pololu board and take just another BrickPi then instead, but either if I have three BrickPi3 + 1 Pololu or all over four BrickPi3, I don't get actually 1 step closer to a final solution, given that BrickPis are not stackable by stack-through pin headers plus software drivers.
Actually the Pololu motor HAT is capable of driving my 2+2 propulsion motors parallely.
My model is a tank-like Tetrix+Matrix chassis, I just finished my first RC driven test drive (a Arduino Due used in the dashboard for BT RC+telemetry).
project discription: http://www.mindstormsforum.de/viewtopic.php?f=70&t=8851
For that test it was just capable of RC, but in the end it will be designed for autonomous operation as well (subsumption architecture).
The Sharp IR sensors are planned for the sides (2*2), back (2) and claw(2),
the USS are for the front sides.
1-2 extra motors perhaps for the USB cam.
The sensors may be attached by PCFs or MCPs, or even the extra Arduino Mega or Due muxer board inside, the Arduino is also designed to run a poszyx positioning board (for inside the house) and the GY-NEO GPS (for outside),
but eventually all that depends on the extra hardware which is still missing:
i.e. a 6 DOF robot arm to be mounted on top and it's motors controlled by the Pi. It will probably look similar to the one I already have been using for my Chess robot, somehow enhanced though:
Unfortunately 1 Arduino will not be capable of doing all the required IO control.
The only alternative setup might have been by Adafruit which builds the only stackable motor HATs I know, but they rediculously don't provide a C API for them, and Lego sensors are too weird by design to plug them directly to PCFs, MCPs, or even to Arduinos, and require too much efforts to get them working p+p.
That's why I was curious about stacking several additional BrickPi3s on top of my Pi and then control all and everything by them.
So having said that I must regrettably admit that the BrickPi is not supposed to help me further unless multiples of them will be stackable by design.