Should I wire my electrical actuators in series, parallel or a combination of both?
If your actuators are DC brush motors, the question of when to wire in series and parallel will depend upon how you need things to behave.  Motors wired in parallel will tend to run at the same no-load speed, which will fall off somewhat with applied torque; motors wired in series will produce the same torque, which will fall off for all motors in the series based upon the total of all the speeds of all the motors.

To be a little more precise, a DC brush motor may be pretty accurately modeled as an ideal motor in series with a resistor and an inductor.  At any moment in time, the rotational speed of the motor will be directly proportional to the voltage across the "ideal" part of the motor, and the torque on the motor will be proportional to the current.  These are both bidirectional relationships, so changing the speed will change the voltage (on the ideal part of the motor) and vice versa; likewise changing the current will change the torque and vice versa.  While this model is not absolutely precise, the difference between most practical DC brush motors motors and an ideal model is in fact very nearly equivalent to a series resistor and inductor.

If your actuators are physically connected such that they will run at the same speed, wiring them in series will balance the torque on them.  If they are not connected that way physically but instead should run near the same speed, wiring them in parallel will cause them to do so, though if they subjected to differing amounts of torque that may cause the rotational speeds to differ somewhat.

If you think in terms of the model described above (and if you're running things with unmodulated DC you may be able to ignore the inductance part) it should be pretty clear what is happening and how you need to wire things.