The two common methods of operating a rotary table is the axis always active, if you want to interpolate the rotary with the linear axis, the other is to disable the servo at position and use a brake, (non-interpolated machining).
The former requires more torque due to machining pressure.
If you figure out the maximum rotation speed you require and then calculate the maximum reduction you can use, based on the max RPM of your motor type, DC servo's usually max out at 3000rpm 4000tops.
Remember reduction multiplies the torque by the ratio of the reduction.
The other thing to consider, is, if you have decided on a reduction and the encoder is on the motor, you will need to select a resolution that is within the reading (speed) capability of the system you are using.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.