I don't see how you can avoid having an ESC per motor. It's not like you can multiplex them.
James Brouton's YT channel has some of the best examples of quadrapeds (and many other odd kinematics) I've seen. https://www.youtube.com/channel/UCUbDcUPed50Y_7KmfCXKohA
His work on OpenDog is very well documented and open source.
Your choice of encoders is going to be driven more by what fits -- there's plenty of relatively cheap encoders these days that have over a thousand steps/rev. You'll have to choose if you're attaching them to the motor directly, or further down the gear train.
A Pi usually isn't going to have the speed to run these motors -- the control loops need to be much tighter. Most often, a Pi or equivalent is used as a supervisory control (keeping the legs coordinated with each other) while the motors are directly run by smaller, higher-speed microcontrollers.
