No, this specific model is designed for the 19mm Studio rod standard. You would need a 15mm-to-19mm adapter or the 15mm version of this rig for smaller rod systems.
Yes, the shoulder pad is attached via Velcro, allowing you to shift it forward or backward to perfectly balance the camera's center of gravity.
Yes, any 19mm studio-standard accessory, including follow focuses and matte boxes, can be mounted directly onto the 19mm rods.