Required force depends on arm geometry. Assuming worst case scenario where you're lifting the entire other robot on the end of your arm the force required to start things moving is:

Actuator force = Weight * Lifter ratio / Sin(Theta)

Where:
Weight is how heavy the object to be lifted is
Lifter ratio is (hinge to tip distance)/(hinge to actuator pivot distance)
Theta is the angle between the actuator and lifter arm when closed

That scenario is unlikely to happen but extra force is better.