Can i power Seeeduino v4.2 through USB and Vin from a shield at the same time?
Yes… 
How do you know, i may burn the arduino.
Because this board was mine desige six years ago. 7-12V VIN will be ok.
Great, thank you Bruce.
I put a Pololu Dual TB9051FTG Motor Driver Shield, but i don’t know the range of it’s current sensing.
It’s written that it delivers 500[mV] for 1[A], so i guess that 5[V]=2.5[A].
Is there a way to know what is the maximum voltage that a motor shield is sending through pins 0 and 1, the current sensing pins, or do i have to rely completely on what the shield’s manufacturer writes, or doesn’t write…
No, 500mv/1A means 1A current go through the motor chip will sensing as 500mv signal. So, 2.5A current will sensing 2.5A*500mV=1250mV=1.25V.
5[V]=5000[mV]/500[mV]=10[A]
But on Pololu site only 2.6[A] can pass to each motor
No more than 2.6A is fine.