The 50 and 55 are pretty much the same except the 55 gave up the speed of the 50 for more torque. Dont worry too much about the voltage as most of our little ESC by Dynam and Esky only run about 5 volt anyway.
The 50 is only about 8 oz of torque and thats about as "weak" as you can get. Its pretty much 1/2A plane material.
No need to waste money on a expensive digital servo for the tail if you dont have a gyro that can "talk" digital to either.
I use futaba 3114 on the tail as it is fairly fast and strong and yet small and light.
I didn't use them for the cyclic as they would be a little difficult to mount due to the small physical size and tiny control arms looked puny to me.