jeffmorris:
Do all servos have to be from one company? Can I use servos from two or more companies?You are free to mix and match as most all common hobby servos share the same operating voltage range and controlled by the same type of TTL voltage level pulse width control method.
Why servo horns made for one brand don't fit servos made by another company?
A camshaft for a Ford car will not fit on a Chevy car either. There is no industry 'standard' for servo output shaft attachment method or size, you get what you get when you select the servo.
Lefty