Yes it is possible to connect a Sabertooth to a Raspberry Pi. Just set the Sabertooth to serial mode using its dip switches and send serial packets from the Pi. You will need to write this code yourself.
As for whether you need a Sabertooth with the Pi, the answer is it depends on what you’re trying to do. If you need to control motors then you need some sort of motor controller (such as the Sabertooth) since you won’t be able to output enough current directly from the Pi’s GPIO to spin the motors.
Hi..as per my knowledge the best way to do this would be an add-on board; there are loads for the Pi and the Sabertooth provides lots of options. You could use simple PWM smoothed by capacitors to give an analogue voltage to each motor (S1 and S2). In this mode, 2.5V is stop, 0V is full speed backward and 5V is full speed forward. It would take a bit of trial-and-error to get the voltages right, and if your motors are big enough to need a 25A driver then a trial-and-error approach might be a little bit dangerous.
But there is an easier option. I use a servo-controller board that outputs 5V RC, because I want to switch between Pi control and manual radio control. This way the servo-controller outputs exactly the same signals you would get from the receiver in a radio-controller toy car and the Sabertooth interprets them in the same way.